68 THE SEAS 



{Gonoplax), which has a reddish-brown rectangular shell 

 and eyes on the ends of movable stalks. 



A great host of molluscs find a home on the sea bottom, 

 the most conspicuous being large snails such as the whelks, 

 which include the common Buccinum, the smooth-shelled 

 Fusus and Sipho, and Neptuna, with bold ridges running 

 round its shell, the two latter being especially northern 

 beasts. The shell of these, and similar, animals is fre- 

 quently covered with a moss-like growth which on examina- 

 tion proves to be a very interesting hydroid called Hydrac- 

 tinia, which has three kinds of polyps, for feeding, repro- 

 duction and defence, respectively. Our largest sea slug, the 

 Triton {Triionia), is frequently found at these depths ; it 

 is pale coloured of various shades and browses on Dead- 

 men's-fingers ! The boat-shell (Scaphander) ploughs its 

 way through sand in the search for the small bivalves on 

 which it feeds. Another strange beast L'ving in sand is the 

 Elephant's Tooth, our sole representative of a small but 

 distinct group of molluscs, which, as its name suggests, 

 has a long tapering, shell, open at both ends. The empty 

 shells are frequently taken possession of by a small Sipun- 

 culoid worm (never by hermit crabs, which always live in 

 spiral shells), which closes up the main opening except for a 

 small hole through which it protrudes its long proboscis. 

 The varieties of bivalves are legion, from the large, very 

 thick-shelled Cyprina islandica of the North Sea to the many 

 smaller kinds which are often almost incredibly abundant. 

 Thus on the Dogger Bank one such, named Spisula, occurs 

 in patches often fifty miles by twenty and at a density of 

 anything from one thousand to eight thousand to the square 

 metre, while another, called Mactra, is found in patches of 

 fifteen to twenty miles in diameter and up to seven hundred 

 per square metre. Everywhere abundant are the many 

 different kinds of Scallops, varying in size from the large 



