ISOPLEURA. 



387 



SUB-CLASS 1. ISOPLEURA* (PLACOPHOKA). 



Symmetrical Gastropoda, icitli metameric repetition of the ctenidia 

 and ht'//-pfates. 



In this sub-class the head is without tentacles and sense-organs. 

 The foot occupies the greater part of the ventral surface, and has 

 a broad flat sole (in Cryptoplax the foot is narrow). The mantle 

 occupies the whole dorsal surface, and completely hides the head. 

 The visceral sac is flattened and not drawn out into a dome. The, 

 mantle-fold encloses a shallow groove which completely surrounds 

 the body, and is roughly of uniform depth throughout. The 

 ctenidia, which are bipectinate and projecting, are attached to the 



PIG. 305. A, Plaxiphora carpenteri from 

 above (4:1). B, a Chiton from below, 

 after Cuvier. C, Lcjitoi-liiton Ldkmi]il 

 from the left (4:1), after Haddon 

 (from Simroth). 



floor of this groove, generally in the hinder part, but in some 

 forms they extend forwards nearly to the head ; they vary in 

 number from six to eighty pairs. 



The shell-plates, of which there are eight, are partly, and in 

 Gi'yptocliiton wholly, embedded in the mantle ; each plate is over- 

 lapped by the plate in front, except in Cryptoplax (CJiitonellus), in 

 which the plates do not touch (Fig. 306). The parts of the mantle 



* A. Th. Middendorf, " Beitrage zu einer Malacozoologica rossica," M6m. 

 Acad. Imp. Petersburg, 1848. A. Sedgwick, "On certain points in the anatomy 

 of Chiton," Proc. Roy. Soc., 1881. H. Simroth, " Mollusca" in Bronn's 

 Klassen u. Ordnuncjen, 1893-94. I. Blumrich, "Das Integument der Chitonen," 

 Z. /. w. Z., 52, 1891. A. C. Haddon, "Report on the Polyplacophora," 

 Challenger Reports, Pt. 43, 1886. 



