THE AMPHINEURA 41 



phylum Mollusca by Spengel, Hubrecht, Lankester, etc., and even 

 von Jhering has since admitted this interpretation. Although 

 Gegenbaur and Glaus have again separated Chaetoderma and 

 Neomenia from the Chitones, and placed the former in a distinct 

 class (" Solenogastres "), and the latter once more in the Gastro- 

 poda, the unity of the Amphineura, as well as their molluscan 

 nature, is now very generally accepted. 



Definition. The Amphineura are a group of Mollusca 

 characterised, firstly, by their more or less elongated and quite 

 symmetrical body, with the mouth and the anus situated at its two 

 ends ; and secondly, by their mantle, which is always provided 

 with numerous spicules embedded in a cuticle. 



General Description. The mantle is very large, and always 

 covers at least the dorsal surface and the sides of the body. The 

 whole external symmetry reappears in the various internal organs. 

 In the nervous system there are, on each side, two longitudinal 

 cords (one pedal, one pallial) with ganglionic cells along their 

 whole extent. They are united with one another in front, where 

 there is a supra-oesophageal cerebral commissure. The two pedal 

 cords are also united by anastomoses, and in addition each of 

 them also exhibits anastomoses with the corresponding pallial 

 cord. The two pallial cords are united by a thick posterior com- 

 missure on the dorsal side of the rectum. There are no otocysts. 

 The buccal cavity is very generally provided with a radula (but 

 mandibles are only present in a species of Chaetoderma). The 

 anus and renal orifices are posterior. The heart is dorsally situated 

 in the hind part of the body, and its ventricle is more or less 

 intimately united to the dorsal wall of the pericardium. 



All the Amphineura are marine in habit. They are found in 

 all oceans and at nearly all depths. They existed in very ancient 

 geological ages, for they are already present in the Ordovician 

 (Lower Silurian). 



There are two very distinct Orders of Amphineura: (1) the 

 Polyplacophora, (2) the Aplacophora. 



ORDER 1. Polyplacophora, Blainville. 



Definition. Amphineura, whose chief characteristics are (1) the 

 foot, occupies the whole ventral face of the body ; (2) the 

 mantle, bears eight transverse calcareous plates ; (3) between 

 mantle and foot there is on each side a more or less complete row 

 of branchiae. 



I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. 



The mantle covers the whole body on the dorsal side; its 

 ventral extension is inversely proportional to that of the foot, and 



