M O L L U S C A. 



621 



-. cle on each side. The anus is placed on the right 

 v^-^.-^/ side. 



3. PATELLA. Limpel. This genus differs from the 

 others of this order, in being covered on the back by a 

 conical shell, within the cavity of which the animal is 

 capable of withdrawing itself. The cloak is large, 

 covering both the head and foot. It is united with 

 the shell along its superior margin. The foot is fleshy, 

 and furnished with numerous muscular filaments, which 

 unite, in the superior part of the cloak, to form a 

 strong muscle, by v hich the body adheres to the 

 shell. The action of this muscle brings the shell close 

 to the surface to which the foot adheres, or removes it 

 to a distance. 



The head is furnished with a large fleshy snout, 

 supporting at the base two pointed tentacula. The 

 eyes are placed on a small elevation at the external 

 base of the tentacul.i. A little way behind the head, 

 and below the cloak, on the right side, are two aper- 

 ture', being the anus and orifice of generation. The 

 gill* occupy the same position as in the preceding ge- 

 nera. In some the branchiae form complete circle ; 

 in others the circle is interrupted anteriorly at the 

 bead. 



Within the trunk, the mouth is fortified by two car- 

 tilaginous cher k, which, at their union, anteriorly, sup- 

 port the base of the tongue. This last is a most sin- 

 gular organ. It is longer than the whole body, narrow, 

 and covered with three rows of short reflected spines, in- 

 terrupted longitudinally and transversely. Its fixed end 

 only is exercised, its free end being coiled up the abdo- 

 men. On the upper side of the mouth is a semicircular 

 osseous plate, or upper jaw. The gullet is furnished with 

 a dilatable pharynx. The stomach is elliptical, with the 

 cardia and pylorus at opposite extremities. The in- 

 testine* are variously folded, and are several times long- 

 er than the body. The salivary glands are minute. 

 The liver U intimately united with the stomach and 

 intestines. 



The heart is situated on the left side, in the anterior 

 part of the body. The auricle receives the aerated 

 blood from one vein when the circle of the gills is 

 complete, and by two when interrupted. This auricle 

 is placed on the anterior side of the heart. An aorta 

 ante* from each tide, to convey the blood to the 

 body. 



The ovarium is placed underneath the liver; and as 

 it exhibits tome differences of organization, M. Cuvier 

 infers that it likewise contains the male organs. 



The species belonging to this group are numerous, 

 and appear to admit of distribution into those having 

 the branchial circle complete, and those in which it is 

 interrupted. 



kite*. * CHITON. This genus is well characterized by a 



cries of testaceous dorsal plates, and the absence of 

 tentacula. 



The body it elliptical. The cloak is firm and car- 

 tilaginous, and variously marked on the margin. The 

 dorsal plates are arched, and occupy the middle and 

 tide* of the back. They are implanted in the cloak 

 in an imbricated manner, the posterior margin of the 

 fir-t valve covering the anterior margin of the second. 

 The foot is narrow. The month is surrounded with a 

 ejemicircular curled membrane. The anus consist* of 

 a short tube, placed at the posterior extremity of the 

 foot, which supplies the place of tentacula. The ex- 

 ternal orifice of generation has not been ascertained. 



The mouth it capable of forming a short proboscis. 

 The tongue ia short, and armed with strong reflected 



spines. The gullet is short, and the stomach, which Molluscs. 

 is lengthened and folded, is membranaceous. The "" ~S~~*' 

 intestine is several times longer than the body, and 

 much folded. The liver is divided into numerous 

 lobes, and intimately united with the stomach and in- 

 testines. 



The heart is situated at the posterior part of the 

 body. The auricle is placed posteriorly, and receives 

 the aerated blood from two veins. Each vein descends 

 along the base of the gills, collecting the aerated blood 

 from the particular side of the body to which it be. 

 longs ; and, what is most remarkable, when opposite 

 the ventricle, it suffers an enlargement, and sends oil' 

 a branch which communicates with it, and again con- 

 tracts and unites with its fellow from the opposite side 

 to form the auricle^ A single aorta arises from the an. 

 terior side. 



The ovarium is conical, and divided into numerous 

 lobes. Behind, two ducts seem to arise, and to pro- 

 ceed one to each side ; but it has not been determined 

 whether they open externally. No male organs have 

 been detected ; nor is there any thing accurately known 

 with regard to the peculiar nature of their hermaphro- 

 ditism. . 



There are several species of chitons described in the 

 article CONCHOLOOY as natives of the British shores. 

 Even these furnish characters by which they may be 

 distributed into groups. 1. Marginal band of the 

 cloak, with tufts of spines, as the C jaicicularit. 2. 

 Marginal band rough, as C. margiiialin ruber and ' 

 neretit. 3. Marginal band striated, as C. Ictvis and alliut. 

 4. Marginal band smooth, as C. leevigatut. The first 

 ought to constitute a new genus apart. 



The two first genera of this order, Phyllidia and 

 Dephyllidia, are destitute of shells, and constitute the 

 order Pleurobranchia of Cuvier. The two last, Patel- 

 la and Chiton, are furnished with shells. They form 

 the order Cyclobranchia of Cuvier. We have ventur- 

 ed to unite them under the latter denomination, being 

 persuaded that the only character common to all of 

 them is derived from the position of the gills, and that 

 Patella and Chiton differ as much in the structure of 

 their other organs from each other, as they do from 

 either Phyllidia or Diphyllidia. 



The only animal in this order which is valuable in 

 an economical point of view, is the limpet. Although 

 used by the ancients as an article of food, it is seldom 

 brought to market in this country. The inhabitants 

 along the coaat, however, frequently use it as food 

 when boiled ; and its juice obtained by this means, 

 when mixed with oatmeal, forms a dish known in Scot- 

 land by the name of Limpet Drose. It is in season 

 about the month of May. It is much used as a bait ia 

 catching littoral fish. 



III. ORDER. Ttclibranchia. 



This order may be considered as represented by the Tcrtibran. 

 genera Aplysia and Bulla of Linnaeus. In many of chia, 

 the order Nudibranchia, the respiratory organs form a 

 double row on each side the back. In the Cyclobran- 

 chia, the same organs occur on each side, but under- 

 neath the cloak. In the present order, the branchia 

 occur underneath the cloak, but they are confined to a 

 particular portion of the right side. They are all in- 

 habitants of the sea. They exhibit the same henna- 

 phroditism as the Nudibranchia. The order consists of 

 the following genera. 



1. PLEUROBRANCHUS. This genus ia particularly Picuro- 

 characterued by the foot and cloak forming two broad brancbus. 



