MOLLUSCA 251 



assist the return of the blood to the heart. The accompanying 

 diagrams (Figs. 112, 113) will help the student follow the main 

 facts of the circulation. In lamellibranchs the ventricle often 

 surrounds the intestine. The corpuscles are colorless and 

 amoeboid. The plasma, however, quite commonly contains a 

 bluish pigment (hcemocyanin) which assists respiration in much 

 the same way as the haemoglobin of the vertebrates. 



293. Excretory Organs. In mollusks the excretory organs 

 consist, when reduced to the simplest terms, of one or more 

 nephridia which communicate interiorly with the pericardium 

 or principal ccelomic space, and with the exterior by way of a 

 tubular ureter. The kidney portion of the tube is much 

 modified, has glandular walls and is well supplied with blood 

 vessels. It lies in the immediate region of the pericardial 

 chamber in most cases. 



294. Nervous System. The nervous system of mollusks 

 is usually made up of at least three pairs of ganglia: (a) the 

 "brain" or cerebral ganglia dorsal to the mouth and varying 

 in size according to the degree of development of the head; 

 (6) connected with the brain by a pair of connectives are the 

 pedal ganglia lying ventral to the mouth and innervating the 

 foot; (c) the pi euro- visceral ganglia variously situated in the 

 different groups and connected with the brain or both with the 

 brain and the pedal ganglia. From it nerves pass to the mantle, 

 and to the posterior organs. In gasteropods and cephalopods 

 all these ganglia are much closer together and are collected 

 about the mouth. Still other ganglia are often associated 

 with them. The student should notice how this collection of 

 nervous matter accompanies the development of "head" 

 organs in the better developed members of the phylum. 



295. The Organs of Special Sense. As usual, scattered 

 sensory cells are situated in the exposed epithelial surfaces. 

 These .give rise to a diffuse sensitiveness to tactile and chemi- 

 cal stimuli. The edges of the mantle and the tentacles are 

 especially sensitive. Patches of sensory cells osphradia 

 are often found near the bases of the gills, which probably 



