166 



of a ventricle and two auricles. Respiration is by gills 

 or ctenidia. (See page 174.) Terrestrial forms breathe 

 through an air chamber formed by the mantle cavity. 



The central nervous system consists of cerebral ganglia 

 on a circum-oesophageal ring which may be drawn out 

 into ventral cords or commissures connecting the inferior 

 ganglia (pedal and pleura)) ; there is also a visceral system 

 of two commissures with ganglia. Eyes (simple or complex) 

 are generally present. 



Molluscs are dioecious (sexes separate), or hermaphrodite. 

 Some are viviparous (e.g., PalwUna). The development is 

 direct (e.g., HeHx), or indirect and involving two larval 

 stages, the trochosphere and the characteristic veliger. 

 Most Molluscs are aquatic, and the majority are marine 

 (littoral, pelagic, abyssal). 



THE PRINCIPAL CLASSES OF THE PHYLUM MOLLUSCA. 



With Examples and Notes, 



Class GASTEROPODA. Most are asymmetrical. The head is distinct 

 and usually bears tentacles. There is an odontophore with a radula. 

 Typically, the " foot " is adapted as a sole on which the animal 

 creeps ; and it is generally has, at the posterior end, a hard oper- 

 culum for closing the opening of the shell when the creature with- 

 draws inside. The mantle is a continuous fold completely en- 

 oircling the body behind head and " foot." The shell, when present, 

 is composed of one piece (univalve), or of more than two pieces. 

 Examples, Chiton. Bilaterally symmetrical. Shell 

 consists of eight overlapping plates. 

 " Foot " occupies the whole ventral 

 aspect of body. Gills paired, a row 

 along each side between mantle and 

 " foot." Marine (littoral). 

 Neomenia. Worm -like ; without a shell 

 and a " foot," but with a ventral ciliated 

 groove. The body completely invested 

 by the mantle which has numerous 

 limy spicules. Radula absent. Marine. 

 Ha (Limpet). A univalve conical 

 shell. Head with one pair of tentacles. 

 Ctenidia replaced by secondary gills 

 around the " foot," between it and the 

 fringed mantle. Marine (littoral). 



