148 MANUAL OF ANIMAL BIOLOGY 



almost completely fused so that externally the double nature is 

 not clearly apparent. From each of the cerebro-pleural ganglia 

 there is also another nerve cord, the cerebro-visceral con- 

 nective, which runs posteriorly and connects with the visceral 

 ganglion situated on the ventral side of the posterior adductor 

 muscle. The visceral ganglion has the same general structure as 

 the pedal ganglion. These three paired ganglia with the con- 

 necting nerves constitute the main parts of the nervous system 

 of the Clam. Peripheral branches are given off from the various 

 ganglia, which innervate the near by organs. 



Sense Organs. In the Clam the sense organs are not so well- 

 developed as in the Arthropoda. Possibly this is due to the fact 

 that the Clam does not move rapidly and, therefore, does not 

 have the same need of complex sense organs. It is interesting 

 to note in this connection that in another class of the Mollusca, 

 which includes free-swimming species like the Squid (fig. 9) and 

 the Octopus, highly developed sense organs, such as eyes, are 

 present. There are, however, in the Clam, two sense organs which 

 should be noted. The first of these, known as the osphradium, 

 consists of a small area of pigmented epithelial cells which covers 

 the visceral ganglion. It is believed that this organ, in some way, 

 is able to determine the purity, possibly the oxygen content, of 

 the water which is drawn into the mantle cavity. If the water 

 which comes in is unsuitable, it presumably stimulates the sensory 

 cells of the osphradium. Another sense organ, the statocyst, is 

 situated near the pedal ganglion and consists of a small vesicle 

 containing a calcareous body. It probably serves as an organ of 

 equilibration. (W. f. 50.) 



TEXTBOOK REFERENCES 



Woodruff, pp. 86-90 ; 273-280. 



Curtis and Guthrie, pp. 238-241 ; 246. 

 Guyer, pp. 138; 144; 678-679. 

 Hegner, pp. 223-249. 

 Newman, pp. 217-230. 

 Shull, pp. 87. 



GENERAL REFERENCES 



Cooke. " Molluscs," in the Cambridge Natural History (Macmillan). 

 Parker and Haswell. Textbook of Zoology. 



