MORPHOLOGY OF THE AMPHINEURA. 215 



the body, and I think there is more probability of this represent- 

 ing a reduced than an incipient stage of the foot-fold of the 

 other genera. 



In ChUonellus the foot is undoubtedly less differentiated (8) 

 than it is in Chiton, where it must certainly already be regarded 

 as homologous with the foot of Gasteropods, &c. 



c. Nervous system. — The most conspicuous feature of the 

 nervous system of the Amphineura is the presence of four longi- 

 tudinal nerve-trunks, united together into one in front of or above 

 the pharynx. The fact of the presence of nerve-cells, intermixed 

 to a considerable extent with the fibrous nerve-matter along the 

 whole course of these trunks, shows that centralisation has not 

 yet by any means reached its limit in this class, but that the 

 whole of the longitudinal stems may, to a certain extent, be 

 looked upon as representing the central nervous system. An 

 anterior cerebral thickening appears to be more marked in the 

 Solenogastres (3, 6, 7, 22) than in the Chitones (1, 8, 20). A 

 posterior coalescence of the four longitudinal stems, or of two 

 of them, into a ganglionic swelling situated above the rectum 

 has been demonstrated with certainty in Chaeioderma (3, 6), 

 Neomenia (4), and Chiton (8a, 20). In Froneomenia there is as 

 jet only probability of its existence (7) ; it will have to be looked 

 for carefully in the first specimens that come to hand. 



The ventral longitudinal stems are united by transverse com- 

 missures in Chiton (8, 20), Neomenia (4, 22), and Proneomenia 

 (7) . The first of these commissures thus closes a ring round 

 the pharynx, which may be called the oesophageal ring. Gan- 

 glionic swellings at the point where the ventral stems commence 

 their backward course, and are united by this first commissure, 

 may be called the infra-oesophageal ganglia. A second, more 

 delicate, nerve-ring round the pharynx has been demonstrated 

 with certainty in Chiton (8, 20), Neomenia {i::), Proneomenia (7), 

 and Ch(Btoderma (4, 6) ; it may be termed the sublingual nerve- 

 ring, and carries a ganglionic swelling — the sublingual gan- 

 glion. 



The transverse ventral (pedal) commissures are placed at 

 regular intervals, and in Neomenia (4) and Proneomenia (7) 

 have been shown to take their course partly through the ventral 

 longitudinal blood-sinus. In the latter genus smaller ramifications 

 have been seen to take their origin from these commissures. In 

 Chiton similar ramifications from the ventral commissures appear 

 to give rise to a plexus-like arrangement of nerve-tissue in the 

 foot (5). 



In Chatoderma similar transverse ventral commissures between 

 the longitudinal stems, although specially looked for, have not as 

 yet been found, and may with a certain amount of probability, 



