MARINE MOLLUSCS 237 



In the Holostomata the shell is either spiral, conical, tubular, or 

 composed of several valves, and the spiral forms are usually closed 

 by a horny or shelly operculum of the spiral kind. The head is 

 provided with a proboscis that is generally non-retractile, and the 

 gills usually extend obliquely across the back, or are attached to the 

 right side behind the head. 



We shall first consider the lower forms, starting with the family 

 Chitonidce, the animals of which, as the name implies, are covered 

 with a shell that resembles a coat of mail. 



Some of these creatures are very common on our rocky coasts, 

 and yet their nature is such that they are liable to be overlooked 

 by those who are not acquainted with their appearance and habits. 

 The shell is oval or oblong, often so coloured as to closely resemble 

 the rocks and stones over which they crawl ; and the animal is so 

 inactive when left exposed by the receding tide, and its flat 

 under surface so closely applied to that on which it rests, that 

 it looks merely like a little convexity of the rock. But after a 

 few have been discovered the eye becomes accustomed to their 

 appearance, and large numbers may be obtained in a short space 

 of time. 



The shell will be seen to consist of eight transverse, curved 

 plates, overlapping each other at their edges, and all enclosed in a 

 leathery mantle, which also forms a projecting margin all round. 

 The middle six plates are different from the first and last in that 

 they are grooved in such a manner that each one displays a dorsal 

 and two lateral areas. 



The animal holds on tightly to the rocks by its large creeping 

 disc-like foot, but may be removed without injury by forcing a 

 knife-blade under the margin of its shell. When examined it will 

 be found that it has not a well-formed head like the majority of the 

 gasteropods, and both eyes and tentacles are wanting. The gills 

 form a series of lamella? round the posterior end of the body, 

 between the edge of the foot and the mantle ; and it is interesting 

 to note that the Chitons further justify the low position assigned 

 to them among the gasteropods by their possession of a simple, 

 central, tubular heart, similar to that of worms. 



Perhaps the commonest of the British species is Chiton cinereus. 

 Its colour is a dull grey, but the ground is variously mottled, often 

 in such a manner as to give it a protective resemblance to its sur- 

 roundings. C. ruber is the largest of our species : its shell is variously 

 mottled with shades of yellow and brown ; C.fctscicularis is bristled. 



