476 THE INVERTEBRATA 



(the aesthetes). 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 varying 

 number of branchial organs, each of which resembles a ctenidium. 

 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 fonvard. 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. 



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- 

 ance. Their cadula may be greatly reduced or even absent. 



Class GASTEROPODA 



Mollusca with a distinct head bearing tentacles and eyes, a flattened 

 foot, and a visceral hump which exhibits the phenomenon of torsion 

 in various degrees and is often coiled; always exhibiting bilateral 

 asymmetry to a certain extent; typically with a shell secreted in a 

 single piece; nervous system with cerebral, pleural, visceral and 

 usually pedal ganglia and a pleurovisceral loop ; a radula ; often a 

 trochosphere larva. 



We can safely say that the Gasteropoda are descended from sym- 

 metrical unsegmented ancestors (p. 471), and that the most prominent 

 differences among their present-day representatives are due to the 

 varying degrees in which they exhibit the phenomena of torsion. The 

 ancestors of the Gasteropoda had not been affected by torsion. They 



