ACEPHALA TESTACEA. 



375 



NucuLA, Lain. — 

 Has the teeth of the hinge in a broken Hne. The form of the shell is elongated and narrowed towards 

 the posterior end. We do not know the animal, but it is probably not much unlike that of the pre- 

 ceding genus. 



For a long time we have placed here the Triyonioi, Brug., so remarkable for their hinge, which is 

 furnished with two plates en chevron, crenulated on both surfaces, and each penetrating into two 

 cavities, or rather between four plates of the opposite side, similarly crenulated on their internal sur- 

 faces. From the marks on the inside of the valves we inferred that the animal had not tubes, of any 

 length at least ; and MM. Quoi and Gaymard having discovered it alive, we find, in fact, that, like 

 the Arcacea;, it has an open cloak without any separate orifices, not even one for the anus. Its foot 

 is huge, truncate, and hooked at its anterior part. The recent Trigoniai resemble the Cockles in the 

 figure of their shell, and in the manner in which it is ribbed. Their interior is nacred. The fossil 

 Trigouia; are considerably different. Their shell is tiattened on one side, oblique, longest in the direc- 

 tion perpendicular to the hinge, and crossed in the contrary direction by series of tubercles. 



THE SECOND FAMILY OF THE ACEPHALA TESTACEA,— 



The Mytilace^, — 



Has the cloak open in front, but with a separate aperture for the passage of excrements. All of then; 



have a foot with which they crawl, or at least draw out, direct, and fix the byssus. They are known 



to the vulgar by the name of Mussels. 



Mussels, properly so called {Mylilus, Linn.), — • 

 Have a closed, triangular shell, with equal ventricose valves. One of the sides of the acute angle forms 

 the hinge, and is furnished with a long, narrow ligament. The head of the animal is in the acute 

 angle ; the other side of the shell, which is the longest, is the anterior one, and allows the passage of 

 the byssus ; it terminates in a rounded angle, and the third side ascends towards the hinge, to which 

 it is joined by an obtuse angle ; near this is the anus, opposite which the cloak forms a peculi;;r ai)cr- 

 ture or little tube. The animal {Callitriche, Poli) has the edge of its cloak provided vvitii branched 

 tentacula near the rounded angle, as it is there that the water required for respiration enters. In front, 

 near the acute angle, there is a small transverse Tuuscle, and a large one behind near the obtuse angle. 

 The foot resembles a tongue. 



In Mi/iiliis, Lam., the summits [of the valves] are nearly terminal. Some species are smooth, others striated. 

 T]ie common Mussel (.1/. ediilis, Linn.) is spread in extraordinary abundance along all our coast, where it is often 

 suspended, in lona: clusters, to rocks, piles, ships, &c. It forms an article of food of some importance, but it is 

 dang^erous when eaten to excess ; [and under certain unknown circumstances, or to some individuals, becomes 

 ileleterious]. Some species have been found in a fossil state, (which Brongniart distinguishes generically by the 

 name Mitiloide). 



In Modiolus, Lam., the apices are lower, and towards the third of the hinge ; they are also more protuberant and 

 rounded, whence the shell has more of the ordinary shape of bivalves. We may also distinguish separately the 

 Lithodomus, Cuv., which has an oblong shell, almost equally rounded at both ends, and the summits very near 

 the anterior. They at first suspend themselves to stones, like the common Mussels, but then they perforate them, 

 aiid bury themselves in the excavations, whence they cannot again issue. After they have made their cells, the 

 byssus ceases to grow.* One species {Mi/tiliis Uthophugns, Linn.) is very common in the Mediterranean, where 

 it furnishes a food agreeable enough on account of its peppery taste. There is another (Modiola caudiyera) which 

 has the posterior end of each valve armed with a very hard little appendage, that is, perhaps, of service in the exca 

 vation of its dwelling.f 



The Fresh-water Mussels {Anodonies, Brug.) — 

 Have the anterior angle rounded like the posterior; and the angle near the anus obtuse, and almost 

 rectilinear: their thin and moderately ventricose shell has no tooth in the hinge, but merely a liga- 

 ment occupying its entire length. The animal {Limncea, Poli) is without a byssus ; and it creeps over 



t The means \)j which the sa.vicavous bivnlrcd Mollusca perforate 

 rocks has ^ven rise to much discussion : some believe that tliey dt; 

 the wiirl< by the mechaniciil action uf the valves ; others attribute it 

 to a solvent secreted by the animal. Ail things con.tidercd, I thii.l< 

 the first of these oidnions, notwilhstanding the dithculliis in the w.-iy 

 of its adoption, is yet the most iirobablc. 



