470 THE ANIMAL KINGDOM. 



SUB-CLASS i. GASTROPODA ISOPLEURA (= Amphineurd}. 



Gastropoda, in which the primitive bilateral symmetry is retained not 

 only in the head and foot, but also in the mantle and visceral dome. The anus 

 is posterior. The antero-posterior axis, joining mouth and anus, is long: the 

 dorso-ventral (dorso-pedal) axis short. The pedal and visceral nerve-cords 

 are straight, parallel with one another, and extend the length of the body : 

 ganglionic enlargements are developed feebly or not at all. The circulatory 

 organs, ctenidia, nephridia, and genital ducts are paired and bi-laterally 

 symmetrical. 



The epidermis consists of a single layer of cells, with a cuticula in 

 which microscopic calcareous spines (Chaetoderma}, spicules, or plates 

 (Neomeniae} are imbedded in connection apparently with the cells of the 

 epidermis. In Polyplacophora (Chiton, &c.) similar calcareous spines and 

 plates, and in some instances chitinous spines, occur on the margins of the 

 mantle, whilst a series of eight calcareous shell-plates covers the median 

 dorsal region. These plates articulate one with the other, and their outer 

 margins are overlapped by a fold of the integument. The foot is narrow 

 and long in Polyplacophora, very narrow and inclosed in a groove in 

 Neomeniae, and only indicated in the posterior part of the body in 

 Chaetoderma. It is ciliated in the two last-named groups, and it has a 

 foot-gland opening below the mouth in Neomeniae. The mantle fold is 

 well developed in Polyplacophora, and surrounds both head and foot. In 

 the Neomeniae and Chaetoderma it is reduced to a collar surrounding the 

 anus, unless the edges of the groove inclosing the foot may be regarded as 

 its homologue as well. The head is not prominent : it is partially sur- 

 rounded by a projecting fold in Polyplacophora, the homologue of the 

 tentacles and cephalic lobes or lips of Anisopleura. The central nervous 

 system consists in the Polyplacophora of a nervous ring situated at the base 

 of the cephalic fold, and connected to two pairs of lateral cords, one pair, 

 the visceral cords, extending down the sides of the body and united 

 posteriorly above the anus ; the other pair, the pedal cords, extending 

 down the median line in the foot. The whole of this system is orange in 

 colour, and consists of both nerve-fibres and ganglion-cells. There is a pair of 

 buccal ganglia and a pair of sub-radular ganglia connected with the cephalic 

 ring. The pedal cords are connected across the median line by numerous 

 and irregular transverse fibrous commissures. The Neomeniae have distinct 

 cerebral enlargements, and in Proneomenia the pedal as well as the visceral 

 cords are connected posteriorly. Transverse pedal commissures are present ; 

 and in Proneomenia transverse commissures between the pedal and visceral 

 cords * Chaetoderma has a bi-lobed cerebral enlargement, from which the 

 pedal and visceral cords take origin. These cords unite together posteriorly, 



1 One or two similar connections have been found in Chiton. 



