3388 



THE MOLLUSKS OR SHELLFISH 



Shipworms generally bore with the grain, only turning aside to avoid a knot or 

 any other obstruction; and although their burrows are almost touching, they 

 seldom appear to run into one another. The animal does not feed upon the wood 

 it excavates, but ejects it in small particles through the siphon. The foot is 

 probably the burrowing organ, but the method of excavation is still imperfectly 

 understood. Hyperotus, Nausitoria, Xylotrya, and Cyphus are other forms of Tere- 

 dinidce; the last named constructing a strong, shelly tube, sometimes a yard long, 

 and two inches in diameter, in which the creature lives buried in the sand. 



SUBORDER Anatinacea 



This, the last suborder of the Eulamellibranchiata, contains thirteen families of 

 which only a few are of general interest. Of the Pandoridas, the typical Pan- 

 dora is distinguished by its compressed, internally pearly shell, which is sometimes 

 semilunate in form; the right valve being flat, and the left somewhat convex. 

 P. incequivalvis is a common British species. In Myadora, an allied genus, the 

 left valve is flat, and the right convex. The species of the third genus, Myochama, 

 attach themselves to other shells by their right valve, the left being ornamented 

 with radiating ridges. Among the other families, Anatina, Thracia, and Phola- 

 domya are interesting genera, the last on account of its rarity in the living condition, 

 and its numerous representatives in bygone ages. Brechites, or Aspergillum, is 

 remarkable, as it is only in the earliest stages of existence that it presents the 

 appearance of a normal bivalve. It subsequently forms an elongate tube, open at 



closed at the 

 other by a 

 frilled disc full 

 of holes, like 

 the rose of a 

 watering pot ; 

 embryonic 

 valves being 

 embedded in 



the surface near the rose. The illustration shows the contracted animal extracted 

 from the shell, (e) indicating the siphonal openings, (c) the mantle aperture, () the 

 anterior side, () the mantle, (d) foot opening. 



Brechites vaginiferus. 



A . Animal contracted, with drawn from shell ; B. Anterior end of the shell. 



Order SEPTIBRANCHIATA 



The members of this group are readily distinguished by the circumstance that 

 the gills are transformed into a muscular septum, extending from the anterior 

 adductor to the separation of the two siphons, and surrounding the foot, with 

 which it is continuous. This septum presents symmetrical orifices. The siphons 

 are sometimes short, or more or less produced. There are two adductors, 

 and the mantle edges are united at three points. There are two families, 



