Some animals may also have cross-connecting vessels in addition to lateral ones. Calcareous spines may be present on the girdle to offer protection from predators. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host.The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 3). The taxon was established as a monophyletic group based on molecular evidence. Mollusks are protostomes. This layer multiplies into a band and then splits internally to form the coelom; this protostomic coelom is hence termed As lophotrochozoans, the organisms in this superphylum possess either a lophophore or trochophore larvae. These clades are upheld when RNA sequences are compared. It is speculated that the eyespots originate from neural tissue and not from the epidermis.Animals in phylum Nemertea show sexual dimorphism, although freshwater species may be hermaphroditic. Phylum Platyhelminthes is divided into four classes. A pair of protonephridia, or primitive kidneys, is present in these animals to facilitate osmoregulation. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 2). The Platyhelminthes consist of two lineages: the Catenulida and the Rhabditophora. Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ. These animals lack a calcareous shell but possess aragonite spicules on their epidermis. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.Figure 4. Calcareous spines may be present on the girdle to offer protection from predators. This leaves the lophotrochozoans.
The overall morphology is laterally flattened, and the head region is poorly developed. This cavity is quite distinct from the coelomic cavity, which in the adult animal surrounds the heart. Antalis vulgaris shows the classic Dentaliidae shape that gives these animals their common name of “tusk shell.” (credit: Georges Jansoone)Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Figure 16. (a) Class Turbellaria includes the Bedford’s flatworm (Pseudobiceros bedfordi), which is about 8–10 cm in length. Animals in phylum Nemertea show a flattened morphology, that is, they are flat from front to back, like a flattened tube. They lack circulatory and respiratory systems, and have a rudimentary excretory system. It is the statocyst that attracts our attention. The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes (Figure 18. Annelids are protostomes. All of the remaining flatworms discussed here are part of the Rhabditophora. Mollusks are eucoelomate, but the coelomic cavity is restricted to a cavity around the heart in adult animals. (credit: Mark A. Wilson)Class Bivalvia (“two shells”) includes clams, oysters, mussels, scallops, and geoducks. Rotifers obtain food that is directed toward the mouth by the current created from the movement of the corona.
Early development in some species occurs via two larval stages: trochophore and veliger. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches and their relatives).Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae (“oligo- = “few”; -chaetae = “hairs”). Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. The primary host is almost always a mollusk. The overall body can be divided into head, body, and pygidium (or tail). The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads that develop (as in polychaetes). Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart associated with it; the mantle has siphonophores that facilitate exchange of water.Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion. The Lophotrochozoa comprise two groups, the trochozoans and the Lophophorata. The feature shared by this group is the lophophore, an … Parasitic forms feed on the tissues of their hosts.
The mastax or jawed pharynx is another structure unique to this group of organisms.The nemertini are the simplest eucoelomates. We would expect a Trp receptor to form a part of a presumptive spring-gated channel, similar to that found in vertebrates.We use cookies to help provide and enhance our service and tailor content and ads. Sexual dimorphism is seen in this class of animals. Two of these adaptations are present in arthropods such as chelicerates and crustaceans (Finally, the third major adaptation that transformed ancestral peristaltic vessels into more efficient pumps was the In summary, bilaterian animals seem to have utilized a host of ingenious adaptations to circumvent the limitations imposed by the ancestral peristaltic design. For example, these toxins may be used to induce paralysis in muscles in specific health applications, similar to the use of botulinum toxin. Parasitic forms feed on the tissues of their hosts. Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like eight-plated shell (Figure 9). See Section 31.1 ( page 636) . They
In many dioecious species, males are short-lived and smaller with no digestive system and a single testis.