MOLLUSCS. 



293 



the vent, enclosing the paired gills. The lingual ribbon is poorly developed, but bears 

 many teeth. The eggs pass out by the renal ducts. The mouth and pharynx can be 

 retracted or extended at will. The second genus of the order is 

 JProneomenia which has been found in the North Sea and in the 

 Mediterranean. It is more elongate and worm-like than JVeo- 

 metiia. Nothing is known of the embryology of either form. 



Order III. — POLYPLACOPHORA. 



Fig. 326, — Neomeiila car- 

 inata, ventral and side 

 views; a, anterior, 6, pos- 

 terior extremity; c, fur- 

 row in whicli tlie foot is 

 concealed. 



The chitons are a group which have made no little trouble 

 for zoologists. In the early days of science they, together with 

 the barnacles (which are really Crustacea) were united in a group 

 characterized by the possession of multivalve shells. More recently they have been 

 assigned a place among the gasteropods, but here they have not 

 been allowed to remain in quiet, almost every author assigning 

 them positions of varjing rank and relationship and one, influenced 

 by their peculiar development and the structure of the nervous 

 system, has actually placed them among the worms. 



In external appearance the most striking feature is the serial 

 arrangement of eight calcareous shells upon the back, indicating 

 a segmentation far from common among the Mollusca. This seg- 

 mentation is carried still farther, and we find the gills similarly 

 arranged on either side of the body (Fig. 328), to the number of 

 sixteen or more, each accompanied by an olfactory organ. Around 

 ^'°" ^^nesTeSa" """**" *^^ margin of the dorsal surface frequently occur calcareous spines, 

 or other forms of ornamentation useful in classification. 

 One of the most interesting of recent discoveries is that 

 the chitons, which have been so long studied and so long re- 

 garded as blind, are (in most genera) really very well pro- 

 vided with visual organs, the whole dorsal surface of some 

 forms being studded with eyes of which not less than eight 

 thousand occasionally exist on a single specimen. These 

 eyes are unlike the dorsal eyes of Onchidium, and like those 

 on the tentacles of Helix, in that the retina is between the 

 nerve and the exterior. These eyes ai-e further shown by 

 Professor Moseley to be developed from peculiar sense organs 

 covering the dorsal surface. No trace of these eyes has yet 

 been found in the fossil chitons. 



The mouth is armed with a well-developed lingual ribbon, 

 in which the teeth are arranged in the following manner, — 

 5. 1. 1. 1. 5; the laterals being large and hooked. The in- 

 testine is coiled in a loose spiral and terminates in a median vent at the posterior end 

 of the body. 



Little is known of the development of the chitons, but that little is very interest- 

 ing. Segmentation gives rise to a true gastrula and at the same time the velum is pro- 

 duced. At the anterior end a single flagellum is produced, which is soon replaced by 

 a tuft of cilia. Shortly a constriction appears behind the velum, and on the dorsal 

 surface appear six or seven transverse plates which may represent the shell glands. 



Fie. 328. —Anatomy of Chiton; 

 ao, aorta; hr, gills; c, ven- 

 tricle; c', one auricle; g, ner- 

 vous ring; o,, mouth; 0(2, ovi- 

 ducts. 



