MOLLUSCA. 247 



testine. The corpuscles are colorless and amoeboid. The 

 plasma, however, quite commonly contains a bluish pigment 

 (haemocyanin) which assists respiration in somewhat the same 

 way as the haemoglobin of the vertebrates. 



287. Excretory Organs. In mollusks the excretory 

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

 more nephridia which communicate interiorly with the peri- 

 cardium 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 peri- 

 cardial chamber in most cases. 



288. 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 pleuro-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 these ganglia are much closer together and are 

 collected about the mouth. Still other ganglia are often asso- 

 ciated with them. The student should notice how this collec- 

 tion of nervous matter accompanies the development of 

 " head " organs in the better developed members of the 

 phylum. 



289. 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 

 have a value in testing the character of the water flowing over 



