TYPE MOLLUSC A. 287 



The nervous system varies in the details of its arrange- 

 ment in the different species, but is characterized in general 

 by a tendency to form ganglia, although nerve-cells are scat- 

 tered along the nerve-cords throughout their entire length. 

 In Proneomenia there is present a well-developed and closely- 

 approximated pair of cerebral ganglia from which arise the 

 pleuro-visceral cords which extend backward along the sides 

 of the body and possess a number of ganglionic swellings 

 near their posterior extremity. Two nerve-rings surround the 

 oesophagus : (1) the cerebro-pedal connectives, which end 

 below in the pedal ganglia, from which two pedal cords extend 

 backward along the foot, in some forms {Dondersia) connected 

 at regular intervals by transverse commissures in an almost 

 metameric manner, ganglionic enlargements of the cords being 

 developed in connection with the commissures ; and (2) the 

 cerebro-buccal connectives, which pass to two buccal ganglia 

 lying below the pharynx. Special sense-organs have not yet 

 been discovered in the Solenogastres. 



The nephridia consist of a pair of tubes which communi- 

 cate internally with the pericardial cavity and, bending around 



Fig. 128.— Diagkammatic Longitudinal Section of Chiton (after Halleb). 

 c = perivisceral coelom. n = nerve. 



7i = heart. p = pericardium. 



m = mouth. ro = reproductive organ. 



1-8 = shell-plates. 



the posterior part of the digestive tract, unite to open into the 

 cloaca ventral to the anus by a common orifice. The walls of 

 the tubes are glandular and probably, therefore, excretory in 

 function, but the nephridia also serve as the ducts for the 

 reproductive elements. With the exception of Chcetoderma the 

 Solenogastres are hermaphrodite, the single reproductive 



