MARINE MOLLUSCS 



225 



is provided with two smaller and separate organs that assist in the 

 propulsion of the blood, which circulates through numerous blood 

 spaces in the bristly mantle. 



About two thousand fossil species of brachiopods are known, 

 extending over a vast range of time ; and the living species, 

 numbering less than a hundred, are found from shallow water to 

 the greatest habitable depths. 



Since the reader is hardly likely to form any extensive acquain- 

 tance with the Brachiopods, we shall illustrate our remarks by the 

 introduction of only one species the Serpent's Head Terebratula 

 (Terebratulina caput-serpentis), which is 

 found in deep water in the North Sea. 

 The interior of the dorsal valve, showing 

 the calcareous loop above referred to, is 

 represented in fig. 154, as is also the 

 exterior of the shell, which is finely 

 striated. The latter represents the dorsal 

 aspect of the shell in order to show the 

 hole in the upturned beak of the ventral 

 valve. 



We have now to consider the large 

 group of head-bearing molluscs (Cepha- 

 lophora), the study of which forms a very 

 important part of the work of the sea- 

 side naturalist; and while we deal with Flo 155. UNDER SIDE OF 

 the general characteristics of this group, THE SHELL OF Natica 

 the reader will do well to have before 

 him a few living typical species in order 

 that he may be able to verify as many 

 as possible of the descriptions here given 

 by actual observation. These types may 

 include such creatures as the whelk, periwinkle, and limpet ; or if 

 marine species are not at hand at the time, the garden snail, fresh- 

 water snail, and slug will serve the purpose fairly well. 



By far the large majority of Cephalopoda are enclosed in a 

 single shell, though a few have a rudimentary shell or none at all. 



As is the case with the lamellibranchs, the shell is composed 

 of both animal and mineral substance, the latter being a calca- 

 reous deposit secreted by the mantle of the animal. The shell is 

 usually spiral in form, as in the whelk, but sometimes conical 

 (limpet) or tubular, 



Q 



catena, SHOWING THE 

 UMBILICUS ; AND OUT- 

 LINE OP THE SHELL, 



SHOWING THE RlGHT 



HANDED SPIRAL 



