Marine gastropods displaying reduction or loss of shell (shell sometimes internal). The head comprises of tentacles and compound eyes. 4. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. 7. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. Classes in Phylum Mollusca Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. They are bilaterally symmetrical, triploblastic and coelomate animal. The phylum Mollusca is divided into the following seven classes: Monoplacophora, Polyplacophora, Aplacophora, Scaphopoda, Gastropoda, Bivalvia, Cephalopoda. Phylum Mollusca. Digestive glands are not distinct; salivary gland is usually present. Examples Achatina, Helix, Umax, Artalimax, Agriolimax (slug) Gonaxis, Lymnaea, Planorbis, Ferrisia,Physa, etc. The shell consists of eight separate pieces on dorsal surface and studded laterally with scales or spines to form a girdle. Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules. Members of class Polyplacophora are better known as “chitons;” these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs). 4. Neometiia is hermaphroditic whereas Chaetoderma is dioecious. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. The gills have evolved into ctenidia, specialised organs for feeding and breathing. The nervous system is primitive, with longitudinal pallial and pedal cords with cross anastomoses. They exhibit organ system level of organization. The morphology of the shell and the underlying animal can vary from circular to ovate. 3.65). TOS4. Examples: Loligo, Sepia, Octopus, Argonauta, etc.. The first two classes are thought to represent primitive molluscs, the last three classes include the great bulk of living species. The shell may be absent or rudimentary; when present may be internal or external, undivided or divided internally into a series of chambers. Their majority live in salt water , some in fresh water and few on land , It is a soft mass , It has a calcareous shell which may be external , internal , absent or reduced , The majority are unisexual and few are hermaphrodites . Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces. Larval stages are trochophore and veliger. Amphineura 3. Asymmetrical molluscs with a spirally coiled mantle and a shell of one piece enclosing a corresponding visceral mass. These animals bear a single conical shell, which has both ends open. 3. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Subclass i. Aplacophora or Solenogastres: 1. Examples: Neometiia, Proneomenia, Chaetoderma, etc. Class 2. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. 3. Calcareous spines may be present on the girdle to offer protection from predators. Shell usually symmetrical, with dorsal hinge and ligament, and closed by 1 or 2 adductor muscles. Classification of Phylum Mollusca. Ordovician to recent, 11,000 living and 15,000 fossil species. They can be found in terrestrial regions as well as in the depths of the seas. 2. This phylum contains about 100,000 described species. 2. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. Largest is Gastropoda. Primarily aquatic gastropods, retaining larval torsion and nervous asymmetry. 2. Amphineura 3. 2. 5. Scaphopoda 5. Mollusks are triploblastic (having 3 embryonic layers), closely related to flatworms/earthworms, and are the first animal phylum we have studied that have a true body cavity; a … The mouth is bounded by two pairs of labial palps acting as lips.
2020 phylum mollusca classes