165 



pressure, open and admit fluid into the tube-foot, bufr 

 prevent fluid from passing back into the radial canaL 

 By this means, and by the contraction of the ampullae,. 

 the protrusible tube-feet are rendered tense. The tube- 

 feet have terminal discs which act as adhesive suckers. 



The primary function of the whole system is locomotor ; 

 but in those Echinoderms (e.g., Ophiuroids and Crinoids) 

 in which the tube-feet are without ampullae and sucker- 

 discs, the function is respiratory and tactile. In certain 

 Holothurians the tube-feet are few and modified (buccat 

 tentacles) for feeding purposes. 



PHYLUM MOLLUSCA. 



State the Distinctive, Characters of the Mollusca. 



The body is not segmented. Typically, the dorsal portion 

 of the integument forms a projecting flap, the mantle, 

 which covers in the space (mantle cavity) occupied by 

 the respiratory organs and which secretes the exoskeleton, 

 a limy univalve or bivalve shell of very variable form. 

 The shell is not closely attached to the body- wall ; and 

 in some instances (e.g., Cuttlefish) it is internal. Part of 

 the ventral body- wall is usually formed into the character- 

 istic muscular "foot," which may be a sole for creeping 

 on (e.g., Snail), or wedge-shaped for ploughing through 

 sand or mud (e.g., Freshwater Mussel), or it may be greatly 

 modified, one part being divided into " arms " with suckers 

 and the other portion forming a tube or funnel through 

 which water is squirted out from the mantle cavity (e.g., 

 Cuttlefish). Most Molluscs (except Pelecypoda) have an 

 odontophore, with a chitinous radula or toothed rasping 

 ribbon, on the floor of the mouth cavity. 



The true body-cavity or coelome is represented by the; 

 pericardium, which communicates with the outside through 

 the nephridium or kidneys. There is an extensive haemo- 

 coele. The blood system is well developed ; the circulation 

 is partly lacunar, i.e., through irregular spaces or sinuses 

 of the haemocoele. Typically, the dorsal heart consists 



