34 NERVOUS SYSTEM. 



for defensive purposes, and it has been suggested by others that 

 it assists carnivorous niollusks like the Murex in boring into 

 the shells of their victims — usually bivalve mollusea. 



The liver in all prosobranchiates is a brownish or greenish 

 gland of extraordinary size, which forms almost the whole of the 

 usually spirallj^ coiled hinder portion of the animal from the 

 stomach back, giving up to the sex-glands but a small space. 

 The form of the liver is, therefore, vei*v much the same as that 

 of the posterior portion of the body itself. It is lobulate, and 

 when removed to water is found to be acinose. The acini at 

 their ends are cleft into many digitiform processes ; the ducts 

 from the acini unite, then those of the lobes, with frequentl}'^ 

 sinus-like dilatations, but ending as two bile ducts, placed one 

 before the other, and which correspond to the largest subdivisions 

 of the liver, and approach and, finally, enter the digestive tract 

 at the stomach ; if a blind sack is present they enter in front 

 of it. One may accordingly regard the liver as a much sub- 

 divided gland, since it is only at a few places, in respect to 

 minute structure, that its ducts and sinuses may be distinguished 

 from the terminal lobes. The liver consists here, as in all 

 univalve mollusks, of an outer structureless membrane and an 

 internal epithelium of roundish secretive cells, which have a 

 distinct nucleus and yellow concretions, and also contain fat. 

 H. Meckel would distinguish fat and bile cells : according to 

 Leydig, liowever there is no such distinction possible. The 

 hepatic lobules are united together by thin inembrane, plexuses 

 of finely subdivided blood-vessels surround them, and externally 

 the whole liver is surrounded by a blood sinus. 



Nervous System. 



The typical arrangement of the nervous system throughout 

 the mollusea is the same, the differences of which we have 

 occasion to speak being simple modifications of a single funda- 

 mental type. The oesophageal ring consists essentially of three 

 pairs of ganglia and a double commissure on each side, and in 

 the more highlj^ organized prosobranchiates these ganglia are 

 approximated and the commissures shortened until the cerebral 

 mass resembles that of the cephalopods, where the ganglia can 

 be distinguished only by the origin of some of the nerves. In 



