AMPHINEURA 549 



cavity perhaps represented by a small cloacal chamber at the posterior 

 end, gills present (Chaetoderma) or absent (Neomenia). 



Craspedochilus is a small mollusc found underneath stones between 

 tidemarks. It looks like an elongated limpet and has exactly the same 

 habits, browsing on small algae and returning after excursions to a 

 centrally situated home. In dorsal view there are seen the eight shell 

 plates which articulate with one another and allow the animal to roll 

 up like a woodlouse. Each plate is composed of two layers, the upper 

 or tegmentum and lower or articulamentum. Both are calcareous, but 

 the tegmentum is traversed by parallel canals containing ectodermal 

 tissue which end on the surface in remarkable sense organs ; some of 

 these have the structure of eyes (the aesthetes). Young individuals, 

 which possess a full equipment of aesthetes are negatively phototropic. 

 As, however, the valves become corroded and covered with encrusting 

 organisms they become indifferent to light. The part of the mantle 

 which surrounds the shells is called the girdle and this contains the 

 spicules which are characteristic of the Amphineura as a whole. 



On the ventral surface is seen the head, which does not project from 

 under the shelter of the mantle. It bears no eyes and no tentacles, and 

 is separated from the foot by a narrow groove. The mantle groove is 

 shallow, running completely round the animal and containing a vary- 

 ing number of branchial organs, each of which resembles a ctetiidium. 

 There may be only six on each side crowded together at the posterior 

 end, or they may occupy the whole groove from the head to the anus. 

 It is probable that the forms with a small number of branchiae are the 

 most primitive, and from the fact that the branchiae are graded in 

 size it seems likely that one of them (the largest) is the original one 

 and the others are derived from it. At any rate the repetition of the 

 branchiae does not mean that the chitons were once metamerically 

 segmented animals. There is no trace of any segmentation of the 

 mesoblast in the larva and there is no correspondence between the 

 numbers of the shell plates and of the branchiae. 



The mantle groove also contains the anus in the middle line 

 posteriorly, on each side, the renal apertures just in front of it, and 

 the genital apertures a little further forward. In this entire symmetry 

 of the various apertures the chitons differ from any living gasteropods. 



The internal anatomy presents the features attributed above to the 

 ancestral molluscs. Another feature which is probably primitive is 

 the uniform distribution of nerve cells in the nerve cords and the 

 consequent absence of ganglionic enlargements. The cords are con- 

 nected by many commissures which form a nerve plexus (Fig. 398 A). 



A point of great interest is the palaeontological antiquity of the 

 group, forms with eight shell valves occurring in the Ordovician. 



The Aplacophora are simplified forms, often worm-like in appear- 



