MORPHOLOGY OF THE AMPHINEURA. 217 



indeed, be said to be absent. It remains a matter of some doubt 

 whether in this genus the nerve-ring which is present round the 

 pharynx (4, 6) is comparable to the oesophageal (4) or to the 

 subhngual ring. I hold the latter view to be the more probable. 

 The four longitudinal stems in Chcetodernia unite, in the hinder 

 part of the body, into two lateral stems, which afterwards coalesce 

 posteriorly in the way above mentioned (3, 6). 



It appears to me tliat the nerve-system of Chmtoderma 

 must be looked upon, not as a more primitive stage, but as a 

 reduction from an arrangement which was originally more in 

 accordance with that of the other Solenogastres. An additional 

 argument for this view will hereafter be gathered from the struc- 

 ture of the intestine and liver. 



rinally, it must not pass unnoticed that in Projieomenia the 

 commissural system offers an increase in compHcation (7), in so 

 far as a series of transverse commissures is present on both sides, 

 uniting the lateral with the ventral longitudinal stems. From 

 these commissures peripheral branches also originate. 



In how far these different facts might eventually be grouped, 

 so far as to throw some light on the phylogeny of certain groups 

 of invertebrates, or of the nervous system in general, has already 

 been more fully discussed by me elsewhere, and may here be 

 safely passed over in silence. 



D. Intestine. — The intestine is simplest in Neomenia and 

 Proneomenia, somewhat more complicated in Chcetoderma, and 

 has attained a far higher degree of specialisation in Chiton. A 

 muscular pharynx is present both in the Solenogastres and in 

 the Chitones. In l^eomenia it is capable of partial protrusion 

 (22). It is internally lined by a chitinous cuticle applied 

 upon a layer of columnar cells, and is variously folded. The 

 cavity containing the radula is in open communication with it. 

 In accordance with the size of the radula this cavity is very con- 

 siderable in the Chitones, very small in Proneomenia and Chceto- 

 derma, apparently absent in Neomenia. Shape and situation of 

 the radula of Chiton have been fully described by different authors 

 (16, 18«, 21). In 1877, when v. Jhering (8) for the first time 

 defined the Amphineuea as a separate group (which, however, he 

 erroneously separated from the Molluscs), he regarded the pre- 

 sence or absence of a radula as one of the chief distinctive cha- 

 racters between the two subdivisions of the Chitones and the 

 Solenogastres (his Placophora and Aplacophora). This distinc- 

 tion broke down when the discovery of Proneomenia (7) showed 

 that in the Solenogastres the radula was not always absent, and 

 that there was even more probability in favour of the view that 

 it was undergoing regressive metamorphosis in this group than 

 that it had not yet been started. The chitinous tooth, which in 



