254 SHELLS AND SHELL-FISH. PART 1. 



running through the body, and of a cartilaginous liga- 

 mentj can be opened and shut like the covers of a book. 

 Upon opening these, the mantle is seen to form a thin 

 covering over all the interior surface of the shell ; upon 

 this being raised, four thin lobes, much smaller, will be 

 discerned, called by Cuvier the " branchial leaflets ;" 

 these, again, envelope the internal parts, as the heart, 

 foot, &c. '' On the sides of the mouth are four trian- 

 gular leaflets," which, according to Cuvier, " are the 

 extremities of the two lips, and serve as tentacula ;" the 

 foot is a mere fleshy mass, and is moved about "by a me- 

 chanism analogous to that of the tongue in quadrupeds." 

 The animal is attached to its shell either by a strong cen- 

 tral muscle, as in the oyster; or by two transverse ones, 

 which cross from one to the other valve, and leave round- 

 ish indentations, generally called muscular impressions. 

 These muscles, however, are of secondary importance, 

 when compared to the remarkable modifications of the 

 animal. In the most typical Dithyra, forming our 

 Macrotbachia, the mantle is so united and prolonged 

 as to form one or two long siphons or tubes by which the 

 food is imbibed; while in the next tribe, Atrachia, these 

 siphons are entirely wanting. The latter animals are 

 also distinguished by a considerable number of the genera 

 having the power of forming a bunch of cartilaginous 

 threads called a hyssus, which they protrude from a small 

 opening between the valves, for the purpose of affixing 

 themselves to marine substances : the whole are herma- 

 phrodites. Such are the two chief or typical tribes of 

 the Dithyra. The three which are aberrant, lose several 

 of these characters, and possess others. In the Tubuli- 

 BBANCHiA, which counect the Dithyra and the Gastero- 

 poda, the structure of the animal is a singular compound 

 of both tribes ; while its tubular shell reminds us of a 

 Teredo fixed on other substances externally instead of 

 internally. The Cheliosomid^, instead of a testaceous, 

 has a coriaceous covering, analogous to the chitons and 

 the tortoises ; while the Branchiopoda, although bivalve 

 shells, are attached by a fleshy peduncle or a strong liga- 



