TYPE MQLLUBOA. 329 



similar peculiarity is found in the Aspergillum. Here, too, the true shell- 

 valves are exceedingly small and are united together by and imbedded in 

 a calcareous tube secreted by the mantle, which projects far beyond the 

 shell proper and is fused throughout the greater portion of its extent. 

 The calcareous tube is open behind for the passage of the two siphons, 

 but anteriorly is closed by a perforated plate, the margins of the perfora- 

 tions being sometimes prolonged into tubes which may branch dichoto- 

 mously. The animal lives imbedded in the sand, the posterior ex- 

 tremity of the shell being directed upwards, and seems to have been 

 derived from forms originally possessing a boring habit, such as is seen in 

 Teredo. 



The foot of the Pelecypoda is as a rule very simple. In 

 the most primitive members of the group, such as Nucula 

 (Fig. 151), it is a flat disk-like structure, recalling somewhat 

 the foot of the Gasteropoda, but more usually it is a keel- 

 shaped structure (Fig. 149, p). The modifications in shape 

 which it undergoes are, however, numerous and it may even 

 in some cases be almost absent, as in the Oyster (Ostrea), but 

 special developments, such as epipodia, are never found in 

 connection with it. A " byssus-gland " is a characteristic 

 development of the Pelecypod foot, consisting of a cavity 

 with usually greatly folded walls lying in the tissues of the 

 foot and connected with the exterior by a canal opening on 

 the sole of the foot. By the cells lining the cavity threads of 

 a horny consistency are secreted by means of which the 

 animal is enabled to fasten itself to stones, etc., or even in 

 some cases, as Mytilus, to move about in the absence of a 

 well-developed foot, throwing out byssus filaments, attaching 

 them, and then drawing itself forward towards them. 



The respiratory organs (Fig. 149, br) of the Pelecypoda 

 consist of a pair of platelike structures situated on each side of 

 the body, and being attached along their dorsal margins hang 

 down between the mantle and the body-wall. Notwithstand- 

 ing their platelike form they are modifications of the plumose 

 ctenidium of the Gasteropods. If the typical bipinnate 

 ctenidium be imagined to be directed parallel to the long 

 axis of the body and the median axis to have fused with the 

 body-wall, so that the two rows of pinnse are bent down so 

 as to lie parallel to one another, the simplest form of the 

 Pelecypod ctenidium, such as occurs in Nucula (Fig. 151), will 



