THE AMPHINEURA 41 
phylum Mollusca by Spengel, Hubrecht, Lankester, etc., and even 
von Jhering has since admitted this interpretation. Although 
Gegenbaur Sand Claus 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 
