THE PHYLUM MOLLUSCA 



247 



Mantle, edi^e 

 Moulh 



B. 



Mantle cavity 

 Radula sheath 

 I > I'Salivary^land 



DiO<^Stive 



eland 

 O 



g^ / / Intestine 



'^'"/ / Reprodaclive pov<z 

 ^^-^^^ Excretory pore 



Alius 



Figure 1 4.3. A, Ventral view of a chiton, with the digestive tract indicated. B, Longi- 

 tudinal section through the mouth showing the radula extending forward over the end 

 of the stitr, cartilaginous tongue. C, The tongue is pushed out and the radula is pulled 

 as far as possible onto its lower surface. D, The radula is pulled posteriorly while the 

 tongue is pressed against the food. After this maneuver both tongue and radula are with- 

 drawn into the mouth. 



The sides of the mantle cavity have several pairs of small gills that 

 hang freely in the water of the cavity. Beneath the edge of the projecting 

 mantle this water is continuous with the environment. 



The amphineuran nervous system is poorly developed. In most 

 molluscs the central nervous system consists of a brain and several pairs 

 of ganglia connected by nerve cords. In the chiton the nerve cells are 

 spread out along cords forming a diffuse system. Such poor centraliza- 

 tion of the nerve cells only reflects the sluggish habit of these animals, 

 and does not necessarily indicate the ancestral pattern of the central 

 nervous system in the phylum. 



106. Class Gastropoda: General Features 



Snails are the only class of molluscs found on land. They also occur 

 in fresh water and in the oceans. Both herbivorous and carnivorous 

 species are found, with appropriate modifications of the radular teeth. 

 Most snails creep like chitons upon a broad muscular foot, but a few use 

 the foot as a lever for jumping while others use it as a fin for swimming. 



The basic feature that distinguishes gastropods from other mol- 

 luscs is the result of an embryologic event, known as torsion (Fig. 14.4). 

 The gastropod embryo develops to a stage known as the veliger. This 

 early embryo is symmetrical, with an anterior mouth and a posterior 

 anus, but at a particular point in its development parts of the body 

 twist or rotate as much as 180 degrees, bringing the anus around (usu- 

 ally to the right) to lie over the mouth. This twist is abrupt and per- 



