1037 



MYTILID^E. 



MYTILIDJi 



1038 



taken for the foot. The true foot, he observes, consists of a muscular 

 tunic more or less thick, which covers the abdomen of the animal, 

 and serves it as an organ of progression ; whilst the organ, which 

 always accompanies the byssus, possesses no character in common with 

 the foot except its mobility. Instead of covering the abdomen as a 

 muscular tunic, it forms a part of the retractor muscle, from which it 

 cannot be separated. At the base of thia organ, with which, when the 

 byssus is torn away, the animal seems to explore the bodies in its 

 neighbourhood, is the sheath in which the byssus is lodged. 



The mantle entirely envelops the animal, and forms three apertures, 

 one of which serves for the passage of the byssus and the ' languette ; ' 

 the second terminates the animal in the siphon ; the third is placed 

 on the back, and gives passage to the excrements. The aperture of 

 the siphon ia elongated many lines in respiration, and can be bent in 

 different directions. 



D. polymorpha. This appears to be the Mytilva Wolgce, Chemn. ; 

 SI. Chemnitzii, FeY. ; M. Hagenii, De Baer ; M. lincatun Waardenburg, 

 and M. Area, Kickx. 



It inhabits seas, lakes, rivers, and marshes; all these conditions 

 seem favourable to it. Dr. Vanbeneden gives the following localities : 

 the Caspian Sea, the Black Sea, and the Baltic, the Danube, the Volga, 

 and the Rhine, where they are found in considerable quantities ; the 

 marshes of Syrmia (the Palatinate), the Canal Quillaume (Belgium), 

 the lakes of Harlem (Holland), the Lea (our river Lea, we suppose, is 

 meant), the Doks (the Commercial Docks, London, probably) and the 

 neighbourhood of Edinburgh (Union Canal) ; so that this form extends 

 nearly over the whole surface of Europe from 43" to 56" N. lat. ; 

 Turkey, Austria, Russia, Germany, Belgium, Holland, and England. 

 (Vanbeneden.) 



Dreitsena polymorpha. 



\, Animal of natural size, showing the siphon exscrted, and the aperture which 

 corresponds with the anus open ; view of the back, a, the siphon ; b, the poste- 

 rior row of papillx ; c, anal aperture. 



2, view of the ventral side, a, the siphon ; !>, the anterior row of papilla; ; 

 f, the languette. 



3, Animal in the left value, a, the abdomen ; 6, the languette in situ ; c, the 

 branchiv ; d, the aperture which gives issue to the excrements ; e, aperture of 

 the byssus ; /, the byssus. Vanbeneden. 



Shell of Dreiaena polymorpha. 

 a, view of inside of valve, showing the septum at the umbo ; 6, the outside 

 of valve. 



Mr. J. D. C. Sowerby appears to have been the first who noticed 

 their introduction into the Commercial Docks in the Thames, to 

 which place he is of opinion that they had been probably brought in 

 timber. 



The species are found at the bottom of the water in beds, like -the 

 marme Mytili, agglomerated in bunches by means of their byssus. 

 They attach themselves to stones, to piles, to other shells (Unto and 

 A nodon), and all the bodies which are in their neighbourhood. Dr. 

 Vanbeneden remarks, in continuation, that they probably often adhere 



;o the keels of boats, and that it is perhaps by such means that they 

 are dispersed over such a considerable extent. 



Modiola (Lamarck). M. Rang makes Modlola the third group ot 

 ;he genus Mylilua. M. Deshayes, in his inquiry whether this geuua 

 ought to be preserved, observes that the Mytili and Modiolce much 

 resemble each other, as all admit, but they offer some difference, the 

 importance of which he proceeds to test. Of the animal he says 

 nothing, their analogy being so perfect, and all their characters, in- 

 ;erual as well as external, being so similar that it is impossible to 

 distinguish them. The habit which certain species have of living in 

 the stones which they pierce has not changed their organisation ; and 

 of the justice of this remark those interested in the subject may, he 

 states, assure themselves by a comparison of the animals themselves, 

 which are abundantly spread abroad in all seas. It is a received 

 principle among all zoologists, he continues, that animals having the 

 same organisation ought to make a part of the same genus ; but as there 

 are persons who attach considerable importance to certain characters 

 in the shells, it is right to reduce it to its just value. The Modiolce 

 differ from the Mytili in not having pointed and terminal umbones. 

 On assembling a great number of living and fossil species of both 

 genera, some will be observed whose umbones, nearly terminal, are 

 overpassed by a small very short border; others in which this border 

 is a little more extended ; and so one passes by insensible degrees from 

 the Mytili to the Modiolce without the possibility of determining the 

 point where one genus ends and the other begins. If the same com- 

 parison is continued between the Modiolce and the Lithodomi the same 

 passage may be observed, and then, concludes M. Deshayes, the 

 observer will be convinced, as we are, of the inutility of these genera. 



M. Rang separates the group of Mytili, consisting of Lamarck's 

 genus Modiola, into two subdivisions. 



* Shell sulcated longitudinally. 



M. sulcctta. Shell bluish-white ; epidermis yellowish ; hinge-margin 

 dentilated. It ia found in the Indian seas. 



Modiola sulcata. 

 a. Shell closed, with byssus ; 6, outside view of valve ; c, inside view of valve. 



** Shell not sulcated longitudinally. 



M. Silicula (including, according to M. Deshayes, M. cinnamomca). 

 Shell marone-brown or whitish ; epidermis deep marone-brown. 



Lamarck gives the seas of the Mauritius as the habitat of M. 

 cinnamomea, and the seas of Australia as that of M. Silicula. 



Modiola Silicvld. 



Lithodomw (Cuv.). Animal oblong, very much elongated, thick, 

 having the mantle prolonged and fringed posteriorly ; anal aperture 

 mesial ; mouth furnished with very small triangular appendages ; foot 

 linguiform, canaliculated, but little developed, and carrying a byssus 

 at its base in the early stages of the animal's life. 



Shell delicate, covered with an epidermis, oblong, very much 

 elongated, subcylindrical, rounded anteriorly, not gaping ; umbo sub- 

 anterior, very distinct; hinge toothless ; ligament linear, for the most 

 part internal, inserted in a narrow and elongated furrow; anterior 

 muscular impression of considerable size. 



L. Dactylus (Modiola lithophaga, Lam. ; Mytilus KMsojpAaj/tw, Linn.). 



In the early stages of life the Lithodomi suspend themselves to rocks 

 and madrepores by means of their byssus; but they soon pierce those 

 bodies in order to introduce themselves, and thus form cavities which 

 they can never leave, in consequence of their increasing volume as 

 they advance in age. The byssus then becomes useless, and is no 

 longer manifested. These shells, says M. Rang, are very common hi 



