BIONOMIC CHARACTERS OF MOLLUSCA 1015 



less. A number of species have an individual range of several 

 hundred fathoms, this range in one of two cases being nearly 

 800 fathoms. 



The mero-planktonic larva of brachiopods is known as the 

 cephalula, and consists of a ciliated umbrella-like anterior end, car- 

 rying four eyes ; a middle portion carrying the 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 de- 

 velops into the body of the brachiopod. 



V. MoLLUSCA, 



Pelecypoda. The pelecypods are marine or fresh-water 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 Car- 

 dium, Solen, Mya, and other marine pelecypods have been obtained 

 in fresh, or nearly fresh, water ; while Unio, on the other hand, has 

 been found in the Brisbane river within reach of the flood tide. 

 In the neighborhood of Rio Janeiro, Solen, and Mytilus, were 

 found living with fresh-water Ampullaria in brackish water (Dar- 

 win-io). 



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 desiccation at low tide. Mytilus cdulis, however, is a good 

 example of a shore pelecypod, for it habitually grows in positions 

 where it will periodically be exposed at low tide ; while Modiola 

 plicatula is especially common in salt marshes, where it is covered 

 only for a short period at high water. The closely related Modiola 

 modiola, which occurs on our northern shores, is, however, seldom 

 exposed, growing either in deep water or in tide pools which are 

 never drained. 



Ostrca arhorca is another striking shore mollusc, growing in 

 vast quantities on the free roots of the mangrove, and withstanding 

 a periodic exposure under a tropical sun. Ostrea horcalis, on the 

 other hand, is at home only in water of several fathoms' depth. 

 The bathymetric range of the pelecypods is very great, and even 

 a single species may have a range of considerable magnitude. 

 Thus, while Mytilus cdulis does not occur below fifty fathoms, 

 another species, M. phaseolinus, ranges from the shore to a depth 

 of 3,000 fathoms. In the greater depths, the pelecypods are com- 



