CHAPTER XIV 



CLASS GASTEROPODA — AMPHINEURA AND PROSOBRANCHIATA 



Order I. Amphineura 



Bilaterally symmetrical Mollusca, anus at the terminal 

 end of the body, dorsal tegument more or less furnished with 

 spicules. 



Sub-order i. Polyplacophora (Chitons). — Foot co-extensive 

 with ventral surface of the body, dorsum with eight transverse 

 plates, articulated (except in Chitonellus)^ a row of ctenidia on 

 each side between the mantle and the foot. Silurian . 



The Chitons are found in all parts of the world, ranging in 

 size from a length of about half an inch to six inches or more in 

 the giant Cryptocliiton. Although in the main sub-littoral, they 

 occur at very great depths : the Challenger dredged Leptochiton 

 benthus Hadd. at 2300 fathoms. Chiton Polii exceptionally 

 occurs at Malta — teste Mac Andrew — above sea margin, but 

 within reach of the ripple. As a rule, the Chitons live in con- 

 cealment, on the under surface of stones or in deep and narrow 

 fissures in the rocks. When the stone to which they are 

 attached is turned over, they crawl slowly to the side which is 

 not exposed, as if disliking the light. An undescribed species, 

 however, which I took at Panama, crawled quite as fast as an 

 ordinary snail. Chiton fulvus Wood, apparently is accustomed 

 to crawl with some rapidity. Mac Andrew took it in abundance 

 on his anchor chain in Vigo Bay every time his yacht was got 

 under weigh. He also found it crawling in sand on the shore, to 

 which habit is no doubt due its extreme cleanness and freedom 

 from the foreign growths Avhich are so characteristic of many of 

 the species. When detached a Chiton contracts the muscles of 

 the whole body, and rolls up into a ball like a wood-louse. 



400 



