MOLLUSCA. 



221 



243. The Organs of Special Sense. Experiments show that the 

 mollusks are sensitive to the usual external stimuli. Sensory cells 

 are scattered over the epithelium of all the exposed surfaces. These 

 give a sense of touch and of the chemical conditions. The edge of 

 the mantle and the tentacles are always especially sensitive. In 

 addition to these are special patches of cells, in many forms, believed 

 to have a smell or taste function. Sac-like cavities, lined with 

 sensory cells and containing a fluid in which are solid particles 

 (otocysts), are found. These have long been believed to be organs 



0- 



FIG. 70. Diagram of the nervous system of the clam from the dorsal aspect, 

 a, anterior; o, mouth; e.g., cerebral (brain) ganglia; p.g., pedal ganglia; v.g., visceral 

 ganglia. 



Questions on the Figure. Find, in the reference texts, a figure 

 showing the nervous system of the clam from the side view. What 

 is the function of each of these ganglia? Compare with figures of 

 other mollusks. 



to perceive vibrations in the medium in which the animal lives, 

 probably sound vibrations; but recent investigations incline us to 

 believe that these structures are rather gravity organs to determine 

 the position of the animal in the water. 



Eyes are usually present, though they vary much in efficiency. 

 Fig- 7 1 gives a series of such eyes, taken largely from the mollusks. 

 The cephalopods have very complex and highly developed eyes, 

 much like those of vertebrates. In the lamellibranchs the eyes, 

 when present, are on the edges of the mantle; in the other types 

 they are about the head. 



