Flatworms, Annelids and Mollusks - OpenStax CNX module: m48096 1 Flatworms, Annelids and Mollusks Miranda Dudzik Based on Superphylum Lophotrchozooa y by OpenStax College This work

OpenStax-CNX module: m48096 1

Flatworms, Annelids and Mollusks*

Miranda Dudzik

Based on Superphylum Lophotrochozoa� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 3.0�

Abstract

By the end of this section, you will be able to:

• Describe the unique anatomical and morphological features of �atworms, mollusks, and annelids• Discuss the advantages of true body segmentation• Explain the key features of Platyhelminthes and their importance as parasites• Describe the features of animals classi�ed in phylum Annelida• Describe the features of animals classi�ed in phylum Mollusca

Flatworms (Phylum Platyhelminthes), annelids (Phylum Annelida) and mollusks (Phylum Mollusca),while extraordinarily diverse, share a number of characteristics, which suggests they are more closely relatedto each other than other invertebrate phyla. Unlike poriferans and cnidarians, all three of these phyla arebilateral and have 3 tissue layers. This means the presence of a head with a concentration of nervous tissues.All of these phyla are also protostomes, or "mouth �rst" animals.

1 Phylum Platyhelminthes

Phylum Platyhelminthes, also known as �atworms include both predatory and parasitic species, includingimportant parasites of humans. Flatworms have three tissue layers. The �atworms are acoelomates, so theirbodies are solid between the outer surface and the cavity of the digestive system.

1.1 Physiological Processes of Flatworms

The free-living species of �atworms are predators or scavengers. Parasitic forms feed on the tissues of theirhosts. Most �atworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather thana complete digestive system. In such animals, the �mouth� is also used to expel waste materials from thedigestive system.Digestion is extracellular. Flatworms have an excretory system with a network of tubulesthroughout the body with openings to the environment and nearby �ame cells, whose cilia beat to directwaste �uids concentrated in the tubules out of the body. The nervous system consists of a pair of nervecords running the length of the body with a concentration of nerves at the anterior end of the worm, where

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there may also be a concentration of sensory cells. These "eyespots" are the �rst rudimentary vision organs,but only capable of distinguishing dark and light, as well as recognize movement.

There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on di�u-sion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constrainingthem to be ��at� worms.

Most �atworm species are hermaphroditic, possessing both sets of sex organs, and fertilization is typicallyinternal. Asexual reproduction is common in some groups in which an entire organism can be regeneratedfrom just a part of itself.

Figure 1: The planarian is a �atworm that has a gastrovascular cavity with one opening that servesas both mouth and anus. The excretory system is made up of tubules connected to excretory pores onboth sides of the body. The nervous system is composed of two interconnected nerve cords running thelength of the body, with cerebral ganglia and eyespots at the anterior end.

1.2 Diversity of Flatworms

Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda(Figure 2). As discussed above, the relationships among members of these classes is being reassessed, withthe turbellarians in particular now viewed as a paraphyletic group, a group that does not have a singlecommon ancestor.



Figure 2: Phylum Platyhelminthes is divided into four classes. (a) Class Turbellaria includes theBedford's �atworm (Pseudobiceros bedfordi), which is about 8�10 cm in length. (b) The parasitic classMonogenea includes Dactylogyrus spp. Dactylogyrus, commonly called a gill �uke, is about 0.2 mm inlength and has two anchors, indicated by arrows, that it uses to latch onto the gills of host �sh. (c) TheTrematoda class includes Fascioloides magna (right) and Fasciaola hepatica (two specimens of left, alsoknown as the common liver �uke). (d) Class Cestoda includes tapeworms such as this Taenia saginata.T. saginata, which infects both cattle and humans, can reach 4�10 meters in length; the specimen shownhere is about 4 meters. (credit a: modi�cation of work by Jan Derk; credit d: modi�cation of work byCDC)

The class Turbellaria includes mainly free-living, marine species, although some species live in freshwateror moist terrestrial environments. The ventral epidermis of turbellarians is ciliated and facilitates theirlocomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrowthe body, even from a small fragment.



The monogeneans are ectoparasites, mostly of �sh, with simple lifecycles that consist of a free-swimminglarva that attaches to a �sh to begin transformation to the parasitic adult form. The parasite has only onehost and that host is usually only one species. The worms may produce enzymes that digest the host tissuesor simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the malegametes develop �rst and so cross-fertilization is quite common.

The trematodes, or �ukes, are internal parasites of mollusks and many other groups, including humans.Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and oneor more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk.Trematodes are responsible for serious human diseases including schistosomiasis, a blood �uke. The diseaseinfects an estimated 200 million people in the tropics, leading to organ damage and chronic symptoms likefatigue. Infection occurs when the human enters the water and a larva, released from the primary snail host,locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cellsbefore reproducing. Many of the eggs are released in feces and �nd their way into a waterway, where theyare able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 3). Tapeworms livein the intestinal tract of the primary host and remain �xed using a sucker on the anterior end, or scolex, of thetapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids,each of which may contain an excretory system with �ame cells, but contain reproductive structures, bothmale and female. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the foodmatter passing them in the host's intestine.

Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point,they are �mature� and all structures except fertilized eggs have degenerated. Most reproduction occurs bycross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of theorganism. The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host andtakes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycleis completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked orpoorly cooked pork, beef, and �sh.



Figure 3: Tapeworm (Taenia spp.) infections occur when humans consume raw or undercooked infectedmeat. (credit: modi�cation of work by CDC)

2 Phylum Annelida

PhylumAnnelida includes segmented worms. These animals are found in marine, terrestrial, and freshwaterhabitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrialhabitats. The name of the phylum is derived from the Latin word annellus, which means a small ring. Animalsin this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms,and leeches. Annelids are protostomes and are often called �segmented worms� due to their key characteristicof metamerism, or true segmentation.

2.1 Morphology

Annelids are bilaterally symmetrical and have a worm-like appearance. Their particular segmented bodyplan results in repetition of internal and external features in each body segment. Metamerism allows animals



to become bigger by adding �compartments� while making their movement more e�cient. The overall bodycan be divided into head, body, and tail. The clitellum is a reproductive structure that generates mucusthat aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fusedband in the anterior third of the animal (Figure 4).

Figure 4: The clitellum, seen here as a protruding segment with di�erent coloration than the rest ofthe body, is a structure that aids in annelid reproduction. (credit: Rob Hille)

2.2 Anatomy

Annelids show the presence of a true coelom. Hence, they are the most advanced worms. Annelids possessa well-developed complete digestive system with specialized organs: mouth, muscular pharynx, esophagus,crop, and gizzard. The gizzard leads to the intestine and ends in an anal opening. A cross-sectional view ofa body segment of an earthworm (a terrestrial type of annelid) is shown in Figure 5s.

Annelids have a closed circulatory system with muscular pumping �hearts� and blood vessels that runthe length of the body with connections in each segment. These animals lack a well-developed respiratorysystem, and gas exchange occurs across the moist body surface. Excretion is carried out by pairs of primitive



�kidneys� that is present in every segment. Annelids have a well-developed nervous system with two ventralnerve cords and a nerve ring.

Figure 5: This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.



:

This combination video and animation1 provides a close-up look at annelid anatomy.

1http://shapeo�ife.org/video/animation/annelid-animation-body-plan



Figure 6: The (a) earthworm, (b) leech, and (c) featherduster are all annelids. (credit a: modi�cationof work by S. Shepherd; credit b: modi�cation of work by �Sarah G...�/Flickr; credit c: modi�cation ofwork by Chris Gotschalk, NOAA)

3 Phylum Mollusca

Phylum Mollusca is the predominant phylum in marine environments. It is estimated that 23 percent ofall known marine species are mollusks; there are over 75,000 described species, making them the secondmost diverse phylum of animals. The name �mollusca� signi�es a soft body, since the earliest descriptionsof mollusks came from observations of unshelled cuttle�sh. Mollusks are predominantly a marine group ofanimals; however, they are known to inhabit freshwater as well as terrestrial habitats. Mollusks displaya wide range of morphologies in each class and subclass, but share a few key characteristics, including amuscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shellof calcium carbonate (Figure 7).

:

Figure 7: There are many species and variations of mollusks; this illustration shows the anatomy of anaquatic gastropod.

Which of the following statements about the anatomy of a mollusk is false?

a.Mollusks have a radula for grinding food.



b.A digestive gland is connected to the stomach.c.The tissue beneath the shell is called the mantle.d.The digestive system includes a gizzard, a stomach, a digestive gland, and the intestine.

Mollusks have a muscular foot, which is used for locomotion and anchorage, and varies in shape and function,depending on the type of mollusk under study. In shelled mollusks, this foot is usually the same size as theopening of the shell. The foot is a retractable as well as an extendable organ. The foot is the ventral-mostorgan, whereas the mantle is the limiting dorsal organ. Mollusks are eucoelomate, but the coelomic cavityis restricted to a cavity around the heart in adult animals. The mantle cavity develops independently of thecoelomic cavity.

The visceral mass is present above the foot, in the visceral hump. This includes digestive, nervous,excretory, reproductive, and respiratory systems. Mollusk species that are exclusively aquatic have gills forrespiration, whereas some terrestrial species have lungs for respiration. Additionally, a tongue-like organcalled a radula, which bears chitinous tooth-like ornamentation, is present in many species, and serves toshred or scrape food. The mantle (also known as the pallium) is the dorsal epidermis in mollusks; shelledmollusks are specialized to secrete a chitinous and hard calcareous shell. Most mollusks have separate sexesand fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails andslugs, or in cephalopods.

3.1 Classi�cation of Phylum Mollusca

Phylum Mollusca is a very diverse (85,000 species) group of mostly marine species. Mollusks have a dramaticvariety of form, ranging from large predatory squids and octopus, some of which show a high degree ofintelligence, to grazing forms with elaborately sculpted and colored shells. This phylum can be segregatedinto seven classes. The most familiar of those classes include Bivalvia, Gastropoda,and Cephalopoda.

Class Bivalvia (�two shells�) includes clams, oysters, mussels, scallops, and geoducks. Members of thisclass are found in marine as well as freshwater habitats. As the name suggests, bivalves are enclosed in apair of shells (valves are commonly called �shells�) that are hinged at the dorsal end by shell ligaments aswell as shell teeth (Figure 8). The overall morphology is laterally �attened, and the head region is poorlydeveloped. Since these animals are suspension feeders, a radula is absent in this class of mollusks. Bivalvesoften possess a large mantle cavity.



Figure 8: These mussels, found in the intertidal zone in Cornwall, England, are bivalves. (credit: MarkA. Wilson)

One of the functions of the mantle is to secrete the shell. Some bivalves like oysters and mussels possessthe unique ability to secrete and deposit a calcareous nacre or �mother of pearl� around foreign particlesthat may enter the mantle cavity. This property has been commercially exploited to produce pearls.



:

Watch the animations of bivalves feeding: View the process in clams2 and mussels3 at these sites.

Animals in class Gastropoda (�stomach foot�) include well-known mollusks like snails, slugs, conchs, seahares, and sea butter�ies. Gastropoda includes shell-bearing species as well as species with a reduced shell(Figure 9). Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by thedigestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species.

2http://openstaxcollege.org/l/clams3http://openstaxcollege.org/l/mussels



Figure 9: (a) Snails and (b) slugs are both gastropods, but slugs lack a shell. (credit a: modi�cationof work by Murray Stevenson; credit b: modi�cation of work by Rosendahl)

: Can Snail Venom Be Used as a Pharmacological Painkiller?

Marine snails of the genus Conus (Figure 10) attack prey with a venomous sting. The toxinreleased, known as conotoxin, is a peptide with internal disul�de linkages. Conotoxins can bringabout paralysis in humans, indicating that this toxin attacks neurological targets. Some conotoxinshave been shown to block neuronal ion channels. These �ndings have led researchers to studyconotoxins for possible medical applications.

Conotoxins are an exciting area of potential pharmacological development, since these peptidesmay be possibly modi�ed and used in speci�c medical conditions to inhibit the activity of speci�cneurons. For example, these toxins may be used to induce paralysis in muscles in speci�c healthapplications, similar to the use of botulinum toxin. Since the entire spectrum of conotoxins, as wellas their mechanisms of action, are not completely known, the study of their potential applications isstill in its infancy. Most research to date has focused on their use to treat neurological diseases. Theyhave also shown some e�cacy in relieving chronic pain, and the pain associated with conditions likesciatica and shingles. The study and use of biotoxins�toxins derived from living organisms�arean excellent example of the application of biological science to modern medicine.



Figure 10: Members of the genus Conus produce neurotoxins that may one day have medical uses.(credit: David Burdick, NOAA)

Class Cephalopoda (�head foot� animals), include octopi, squids, cuttle�sh, and nautilus. They display vividcoloration, typically seen in squids and octopi, which is used for camou�age. The ability of some octopusesto rapidly adjust their colors to mimic a background pattern or to startle a predator is one of the moreawe-inspiring feats of these animals. All animals in this class are carnivorous predators and have beak-likejaws. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well asa closed circulatory system. The foot is lobed and developed into tentacles, and a funnel, which is used astheir mode of locomotion. Suckers are present on the tentacles in octopi and squid.

Cephalopods is facilitated by ejecting a stream of water for propulsion. This is called �jet� propulsion.are able to move quickly via jet propulsion by contracting the mantle cavity to forcefully eject a stream ofwater. Cephalopods have separate sexes. Members of a species mate, and the female then lays the eggs in asecluded and protected niche. Females of some species care for the eggs for an extended period of time andmay end up dying during that time period. Cephalopods such as squids and octopi also produce sepia or adark ink, which is squirted upon a predator to assist in a quick getaway.

In the shell-bearing Nautilus spp., the spiral shell is multi-chambered. These chambers are �lled withgas or water to regulate buoyancy. The shell structure in squids and cuttle�sh is reduced and is presentinternally in the form of a squid pen and cuttle�sh bone, respectively. Examples are shown in Figure 11.



Figure 11: The (a) nautilus, (b) giant cuttle�sh, (c) reef squid, and (d) blue-ring octopus are allmembers of the class Cephalopoda. (credit a: modi�cation of work by J. Baecker; credit b: modi�cationof work by Adrian Mohedano; credit c: modi�cation of work by Silke Baron; credit d: modi�cation ofwork by Angell Williams)

4 Section Summary

Flatworms are acoelomate, triploblastic animals. They lack circulatory and respiratory systems, and havea rudimentary excretory system. This digestive system is incomplete in most species. There are fourtraditional classes of �atworms, the largely free-living turbellarians, the ectoparasitic monogeneans, and theendoparasitic trematodes and cestodes. Trematodes have complex lifecycles involving a molluscan secondaryhost and a primary host in which sexual reproduction takes place. Cestodes, or tapeworms, infect thedigestive systems of primary vertebrate hosts.

Phylum Annelida includes segmented animals. Segmentation is seen in internal anatomy as well, which



is called metamerism. Annelids are protostomes. These animals have well-developed neuronal and digestivesystems. Some species bear a specialized band of segments known as a clitellum.

Phylum Mollusca is a large, marine group of invertebrates. Mollusks show a variety of morphologicalvariations within the phylum. This phylum is also distinct in that some members exhibit a calcareous shellas an external means of protection. Some mollusks have evolved a reduced shell. Mollusks are protostomes.The dorsal epidermis in mollusks is modi�ed to form the mantle, which encloses the mantle cavity andvisceral organs. This cavity is quite distinct from the coelomic cavity, which in the adult animal surroundsthe heart. Respiration is facilitated by gills known as ctenidia. A chitinous-toothed tongue called the radulais present in most mollusks. Early development in some species occurs via two larval stages: trochophoreand veliger. Sexual dimorphism is the predominant sexual strategy in this phylum. Mollusks can be dividedinto seven classes, each with distinct morphological characteristics.

5 Art Connections

Exercise 1 (Solution on p. 18.)

Figure 7 Which of the following statements about the anatomy of a mollusk is false?

a. Mollusks have a radula for grinding food.b. A digestive gland is connected to the stomach.c. The tissue beneath the shell is called the mantle.d. The digestive system includes a gizzard, a stomach, a digestive gland, and the intestine.

6 Review Questions


Annelids have a:

a. pseudocoelomb. a true coelomc. no coelomd. none of the above


Which group of �atworms are primarily ectoparasites of �sh?

a. monogeneansb. trematodesc. cestodesd. turbellarians


A mantle and mantle cavity are present in:

a. phylum Echinodermatab. phylum Adversoideac. phylum Molluscad. phylum Nemertea



7 Free Response


Describe the morphology and anatomy of mollusks.



Solutions to Exercises in this Module

to Exercise (p. 16)Figure 7 Dto Exercise (p. 16)Bto Exercise (p. 16)Ato Exercise (p. 16)Cto Exercise (p. 17)Mollusks have a large muscular foot that may be modi�ed in various ways, such as into tentacles, but itfunctions in locomotion. They have a mantle, a structure of tissue that covers and encloses the dorsal portionof the animal, and secretes the shell when it is present. The mantle encloses the mantle cavity, which housesthe gills (when present), excretory pores, anus, and gonadopores. The coelom of mollusks is restricted tothe region around the systemic heart. The main body cavity is a hemocoel. Many mollusks have a radulanear the mouth that is used for scraping food.

Glossary

De�nition 11: Annelidaphylum of vermiform animals with metamerism

De�nition 11: clitellumspecialized band of fused segments, which aids in reproduction

De�nition 11: mantle(also, pallium) specialized epidermis that encloses all visceral organs and secretes shells

De�nition 11: metamerismseries of body structures that are similar internally and externally, such as segments

De�nition 11: Molluscaphylum of protostomes with soft bodies and no segmentation

De�nition 11: nacrecalcareous secretion produced by bivalves to line the inner side of shells as well as to coat intrudingparticulate matter

De�nition 11: radulatongue-like organ with chitinous ornamentation


Documents

Flatworms, Annelids and Mollusks - OpenStax CNX module: m48096 1 Flatworms, Annelids and Mollusks Miranda Dudzik Based on Superphylum Lophotrchozooa y by OpenStax College This work