74 LECTURES ON MOLLUSCA. 



which are quite hidden ; the creature looking outside only like a lump 

 of leather. 



Another main division of the Chitons contains creatures which have 

 pores in the mantle margin ; always nine on each side, and armed 

 with bristles. The great Plaxiphora of the Cape Horn district has 

 irregular bunches of bristles, some of them shelly. The shells of 

 Acanthochites are beautifully adorned with regular tufts of bristles, 

 which are often of pearly hue. Amiculais almost covered by the hairy 

 mantle, like Cryptochiton. In Cryptoconchus, the tufted pores are at a 

 distance from the edge; and the exposed parts of the valves are 

 extremely narrow. Lastly Cliitonellus has a long, narrow, fleshy, 

 slug-like body, with very small and separate valves, adapted to crawl 

 in the crevices of coral rocks. 



Valves belonging to the Chiton group have been found in most geo- 

 logical periods, from the Silurian age downwards. In one of the 

 Silurian forms, called Helminthochiton, the valves were separate from 

 each other, but not covered by the mantle. 



ORDER CIRROBRANCHIATA. (Tuft-gilled Crawlers.) 

 Family DENTALIADJE. (Tusk-Shells.) 



The tooth-shells form a very peculiar and degraded group, which it 

 is the fashion to arrange near the Key-hole Limpets, from the fancied 

 analogy of the tubular shell to a drawn-out Fissurella. They have 

 however scarcely anything in common with that beautiful family, and 

 very little with the class of Crawlers. The very Vermetids look down 

 upon them ; for they have heads, tentacles, and eyes, while these have 

 none. The animal is scarcely raised above the bivalves, except that 

 it feeds upon them. The foot is conical and funnel-shaped, opening 

 into the stomach, which is armed with a gizzard, as in the Bullas. In 

 fact they belong rather to the Opisthobranchiate division, the fringe- 

 like gills being behind the heart. The blood is red, as in the worms : the 

 breathing organs symmetrical, as in the Chitons. They have however 

 a lingual ribbon, in three series, on a very simple plan. They live in 

 rather deep water, where they prey on Foraminifera and small bivalves. 

 Just as the shell of Vermetus resembles Serpula, so the shell of Denta- 

 lium often might be mistaken for Ditrupa, also a sea-worm. The 

 Ditrupas however generally have a swelling behind the mouth, while 

 that of the tooth-shells is plain. In the group Entalis, there is a slit 

 at the side of the anal hole. Often a small tube is protruded beyond 

 the hole, which is not a constant character, even in the species. 



SUB-CLASS PULMONATA. 

 (Air-Breathers . ) 



We have already passed under review many of the air-breathing 

 mollusks, which by their general affinities seemed more nearly related to 

 the marine tribes. The mere fact of crawling on land rocks and plants 

 instead of river and shore ones, does not necessarily imply any great 



