4i o CoMPARAi^iy-E ANATOMY AND PHYSIOLOGY. 



of the chambered organ and the axial cords appears 

 to be complete.* 



The greater number of the Mollusca present us 

 with an arrangement of the nervous system which is 

 very different from that which obtains in Arthropods ; 

 this is due to the want of metameric segmentation, 

 and to the marked tendency of the ganglionic masses 

 to fuse with one another. Indications of a more 

 primitive condition of things are not, however con- 

 fined to Proneomenia (page 402); commissures con- 

 necting the two chief longitudinal trunks, and so 

 giving rise to a step-ladder-like kind of arrangement, 

 are to be observed in Chiton and in Haliotis. 



In the L,amelliforanchiata (e.g. Anodon), when 

 the primitive bilateral symmetry of the body is re- 

 tained, we find two supraoesophageal ganglia, whence 

 nerve cords pass off on either side to the hinder end of 

 the body ; no ganglia are developed on the course of 

 these trunks, but, as in Proneomenia, at their termin- 

 ations only (visceral ganglia) ; these two ganglia 

 are sometimes almost separate, in other cases more or 

 less completely fused with one another, just as, at the 

 other end of the body, is the case with the supra- 

 oesophageal ganglia. 



These last also give off a pair of cords, which in the 

 mussel extend some way down into the substance of 

 the foot, where they end in the pedal ganglia ; but 

 these pedal ganglia are not always so far distant from 

 the supracesophageal as in the mussel, their size and 

 position depending on that of the foot itself. 



While the supracesophageal or cerebral ganglia of 



* Prof. Milnes Marshall, who has lately repeated and extended 

 the observations of Dr. Carpenter, has suggested that the ant- 

 ambulacral or dorsal portion of the nervous system of a Crinoid is 

 modified from the antambulacral portion of the primitive nerve 

 sheath, which in the starfish still invests the whole of the body. 

 The "chambered organ," or "central capsule," still requires in- 

 vestigation from the morphological and embryological side.. 



