248 ANIMALS OF THE SEA FLOOR 



overlapping as in an armadillo or wood-louse (Fig. 184). 

 Allied to the Chitons is a very remarkable group of 

 molluscs, in which the shell is entirely absent. These 

 are for the most part wormlike in form, and are usually 

 found tightly coiled round the branches of Hydroids 

 and Alcyonarians. 



The nudibranchs, or sea- slugs, are shell-less gastro- 

 pods, which externally appear to be symmetrical. 

 They start life, however, in a minute coiled shell. One 

 section of them, the Dorididae, in which the gills are 

 arranged in a circle on the hinder end of the animal, 

 are found in their greatest development, both in number 

 and size, in tropical seas, where, both along the shore 

 and in shallow water, they attract attention by their 

 large size and brilliant colours. Another section, the 

 Eolididae, in which the gills are not concentrated on 

 one spot, but scattered as papillae all over the body, 

 are, on the contrary, very poorly represented in warm 

 seas, but are richly present on our own coasts (Fig. 185). 



The Lamellibranchs, comprising such molluscs as 

 the oysters, mussels, cockles, scallops, and so forth, are 

 too well known to need further mention. 



Among the Cephalopods, the Octopus and its allies 

 (Fig. 187), with eight equal arms and a rounded body, 

 must be distinguished from the squids and cuttle-fish, 

 which have ten arms, one pair of arms being much 

 longer than the rest. The "paper Nautilus" belongs 

 to the first of these groups. The second includes a 

 great variety of forms — some short and squat, like 

 Sepia, Sepiola, and the cuttle-fish, others elongate 

 like Loligo and the squids (Fig. 186). The pearly 

 Nautilus belongs to a group apart from all the rest. It 

 is the sole representative of a once very abundant 

 Order, to which the Ammonites, so well known as fossils, 

 also belonged. Most of the Cephalopods can swim 



