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MOLLUSC A. 



Molluscs. In the restricted genus Mytilus, represented by the 

 """"Y- 1 "'' common mussel, the beak is terminal. The margin of 

 die cloak, at the rounded angle of the front of the 

 shell, is fringed with branched tentacula. In the ge- 

 rrus Modiolus, represented by the Mytilus modiolus of 

 Linnaeus, the beak is removed a little way from the an- 

 terior extremity. In the Lithodomus of Cuvier, which 

 includes Mytilus lithophagus, both extremities of the 

 shell are equally rounded, whereas in the Mytilus and 

 Modiolus, the anterior is the most pointed. In these 

 three genera there is a foot and byssus. 



The genera Anodonta and Unio are inhabitants of fresh 

 water, resembling each other in the form of the ani- 

 mal, but differing in the structure of the hinge of the 

 shell, the former being destitute of the teeth possessed 

 by the latter. The foot is large and compressed. The 

 posterior end of the cloak is fringed with small tenta- 

 cula. The Anodonta is represented by Mytilus anati- 

 nus, and Unio by Mya margaritifera of Linnasus. M. 

 Cuvier considers the genera Cardita, Venericardia, and 

 Crassatella, as nearly allied to the genera of this sec- 

 tion. 



SECTION III. 



This section includes at present only two genera, 

 Tridactina and Hippopus. The cloak is furnished with 

 three openings, all of them near the anterior extremity. 

 The first of these is the largest, and serves for the 

 passage of the byssus. The second admits the water 

 to the branchiae, and the third is opposite the anus. 

 The valves are closed by one adductor muscle. 



SECTION IV. 



The animals of this section resembles those of the 

 last in the three apertures to the cloak. The first is 

 large, and the two last are sometimes produced into 

 separate or united tubes. The foot issues from the 

 anterior opening, while the two posterior apertures 

 serve for respiration and the passage of the excrement. 

 There are two adductor muscles for closing the shell, 

 one near the mouth, and the other at the anus. 



In the genus Chama, the two posterior apertures are 

 in the form of short tubes, the anterior one is small, 

 and indicates the corresponding size of the foot. The 

 foot of the animals of the Isocardia is much larger, and 

 the anterior aperture is large in proportion. 



In the common cockle, (Cardium) prized by many 

 as an agreeable article of food, the foot occupies a 

 large share of the cavity of the shell. It is bent in the 

 middle, with the point directed forwards. Cockles are 

 considered in season during March, April, and May. 

 They are sold in this country by measure, and eaten 

 either raw, or boiled and pickled. 



In the genera Cyclas, Tellena, Donax, and Venus, 

 the foot is long and tongue-shaped, and the posterior 

 tubes, in general, considerably produced, and more or 

 less united at the base. In the genus Loripes, the foot 

 is small and cylindrical, and the tubes short and united. 

 In the Mactra the tubes are likewise short, but the 

 foot is compressed. 



SECTION V. 



In the animals of this section the cloak is united in 

 front, with an opening at the anterior extremity for the 

 passage of the foot, and a production at the other ex- 

 tremity in the form of two united tubes. The cuticle 

 of the cloak is continuous with that which invests the 

 shell. Hence, when the animal is removed, the cuti- 

 cle frequently remains on the margin of the valves in 

 loose membranes. 



The animals of the different genera are constructed Moiiusca. 

 nearly on the same plan, so that it is impossible, with- * ~ r ' 

 out the aid of the shell, to give them any definite ar- 

 rangement. The following genera are considered as 

 belonging to this section : Mya, Lutraria Anatina, 

 Glycymeris, Panopea, Pandora, Gastrochsna, Bysso- 

 mia, Hiatella, Solen, Sanguinolaria Pholas, Teredo 

 and Fistularia. They all appear to prefer conceal- 

 ment, lodging in the sand, in stones or wood. Many 

 of them are used as food, as the Razor-fish (Solen) 

 and Gapers (Mya.) 



CLASS V. TUNICATA. TCKICATA 



THIS Class was first instituted by Lamarck in the 

 year 1816, and has now been generally adopted by 

 naturalists. It includes the " Acephales sans coquilles" 

 of the system of M. Cuvier, and embraces several of 

 the zoophytes as well as mollusca of the system of 

 Linnaeus. 



The animals of this class are all naked, the external 

 covering being soft and coriaceous. There are two 

 apertures, one for the use of the gills in respiration, 

 and the other for the digestive organs. These aper- 

 tures are frequently surrounded with productions in 

 the form of tentacula. The inner cloak is in many 

 genera loose, in others adhering, but in all it is united 

 with the external one at the two apertures. These 

 sacs are furnished with muscular bands and filaments ; 

 and traces of a nervous system have been perceived. 

 The alimentary canal is very simple, and scarcely can 

 be distinguished into gullet, stomach, and intestine.' 

 The liver adheres to the stomach, and in many is 

 divided into distinct lobes. The gills corer the walls 

 of the cavity of the inner tunic. They are in the form 

 of ridges more or less complicated. The circulating 

 system appears to be reduced to a single systemic ven- 

 tricle. The organs of reproduction consist of an ova- 

 rium, either simple or complicated, with some addi- 

 tional glands whose uses are not ascertained. 



All the animals of this class live in the sea, and are 

 very widely distributed. Some of them are fixed to 

 rocks and sea-weeds, others move about in the water. 

 Many genera possess species whose individuals are 

 detached and independent, while with others there is 

 an inseparable union. These united individuals con- 

 stitute a symmetrical mass, in some cases capable of 

 moving about in the surrounding element. 



The investigation of the structure of this class of 

 animals has been conducted with great care and suc- 

 cess by M. Savigny, in his " Rccherches anatomiques 

 sur Us Ascidies composees el sur les Ascidies simples" 

 inserted in his " Memoires sur les Animaux tans Ver- 

 tebres." Paris, 1816. MM. Desmaret and Lesueur 

 have likewise contributed materially to unfold their 

 structure. The labours of these authors, together 

 with Cuvier's papers on the genera Salpa and Ascidia, 

 embrace nearly all the information which have been 

 obtained regarding the structure and physiology of 

 the animals which have been brought together in this 

 class. It is our intention to give a brief exposition of 

 the systematical characters of the genera, according to 

 the method followed by M. Savigny. 



SUBDIVISION I. TETHYDES. 



IN this subdivision the inner tunic is detached frorri Tethydc* 

 the external covering, except at the two orifices. 

 The branchiae are large, unequal, and cover the walls 

 of the great cavity. The opening to this cavity is 

 surrounded on the inside with a membranaceous, den- 

 ticulated ring, or with a circle of filaments. 



