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The mero-planktonic larva of braehiopods is known as 

 the Cephalnla, and consists of a ciliated umbrella-like an- 

 terior end, carrying- four eyes, a middle portion carrying I he 

 mantle lobes, and a posterior portion. When the larva 

 becomes attached by the posterior end, which develops into 

 the pedicle of the adult, the anterior end becomes enveloped 

 by the forward turning mantle lobes and develops into the 

 body of the brachiopod. 



Pelecypoda. — The pelecypods are marine or fluvial ben- 

 thonic molluscs, which lead either a sedentary or a vagrant 

 life. The majority of species live in the sea, but of these 

 some can adapt themselves to brackish or even fresh water. 

 Thus, species of Cardium, Solen, Mya, and other marine 

 pelecypods have been obtained in fresh, or nearly fresh, 

 water. 



A number of pelecypods inhabit the shore-zone, but the 

 majority of these live buried in the sands and muds, and so 

 are protected from dessication at low tide. Mytilus edulis, 

 however, is a good example of a shore pelecypod, for it 

 habitually grows in positions where it will be periodically 

 exposed at low tide. The closely related Modiola modiolus, 

 which occurs on our northern shores associated with the 

 preceding, is, however, seldom exposed, growing either in 

 deep water or in tide pools which are never drained. 



Ostrea arborea is another striking shore mollusc, grow- 

 ing in vast quantities on the free roots of the mangrove, and 

 withstanding a periodic exposure under a tropical sun. 

 Ostrea borealis, on the other hand, is at home only in water 

 of several fathoms depth. 



The bathymetric range of the Pelecypoda is very great, 

 and even a single species may have a range of considerable 

 magnitude. Thus, while Mytilus edulis does not occur 

 below fifty fathoms, another species, M. phascolinus, ranges 

 from the shore to a depth of 3,000 fathoms. In the greater 

 depths, the pelecypods are commonly characterized by ex- 

 ceeding delicacy of shell and sculpture, the shell being often 

 quite transparent. Some deep-water species show bright 



