618 



M O L L U S C A. 



Molliisca. ther the fluid carried off by this conduit be excremen- 

 "" "V ' titious matter, separated by the liver, or whether the 



nd which produces it be distinct from that organ, 

 but so interwoven therewith as to elude the observation 

 of th* anatomist. 



It is obvious, from the structure of the digestive or- 

 gans, that the species subsist on soft food, requiring 

 neither cutting nor grinding, and in this respect differ 

 remarkably from the species of the genus Tritonia, 

 which were formerly arranged along with them. 



The organs of generation differ little from the other 

 hermaphrodite gasteropoda. The vesicle furnishes two 

 canals, one of which goes to the testicle, the other to 

 the penis. There is likewise a minute bag connected 

 with the canal .of the latter. They deposit their spawn 

 011 sea- weeds and stones. It is gelatinous, of 'a white 

 colour, and in appearance resembles the Spongia com- 

 pvessa. 



The species of this genus are numerous, although 

 their specific characters are still imperfectly ascertain- 

 ed. The following species have been detected in the 

 British seas. 



1. Doris argo. Body about three inches in length, 

 nearly smooth, and of a lemon-yellow colour. The anal 

 plumes about twelve in number. It is called sea-lemon. 

 Pennant's Bril. Zool. iv. p. 4-3. Tab. 22. 



This species is not uncommon in the crevices of the 

 rocks about low-water-mark. 



2. Doris icrrucosa. Body about one inch in length, 

 the cloak closely covered with tubercles ; superior ten- 

 tacula, smooth at the base, compressed at the summit. 

 The vent surrounded with about twenty-four plumes. 



The species which Cuvier has figured under this 

 name, Mem, sur le Genre Doris, Tab. 1. Fig. 4, 5, 6, 7. 

 is represented as having only sixteen anal plumes. 

 Equally common with the preceding in the same situa- 

 tion. Frequently cast ashore after storms. 



3. Doris laii'is. Body about half an inch in length, 

 and widest in front. Foot narrow. Cloak smooth in 

 the middle, tuberculated slightly towards the margin. 

 The vent surrounded with eight plumes. 



This species is figured by Muller, Zool. Dan. Tab. 

 47- Fig. 3, 4, 5. It is very common in company with 

 the former among the Zetland isles. 



-K Doris marginala. Length about a quarter of an 

 inch. Cloak smooth, tinged with pink, with an undu- 

 lated membraneous border, usually four-pointed in 

 front. 



This species occurs on the coast of Devon, where it 

 v.-ss observed by the late George Montagu, Esq. by 

 whom it was described and figured in the Linn. Trans. 

 Vol. vii. p. 79- Tab. 7. Fig. 7. He refers to the figure 

 D. kevis in the Zaologia Danica as synonimous, but 

 with doubt. We have no hesitation, however, in con- 

 sidering them as distinct. The D. electrina of Pennant 

 is, however, probably a variety, but the brevity of the 

 description and imperfection of the figure which, he has 

 given render reference uncertain. 



5. Doris nodosa. Length about half an inch. Foot 

 broai!, emarginate in front. Cloak with four equidistant 

 papilla;, on each side a mesial line. Plumes of the vent 

 about nine in number. 



This is another species discovered by Montagu, 

 Linn. Trans. Vol. ix. p. 107. Tab. vii. Fig. 2. 



6. Doris quailricornis. Length about three-eighths 

 of an inch. Cloak smooth in the middle, with a row of 

 obsolete tubercles on each side. Tentacula approxima- 

 ted in pairs. Vent surrounded by eight or nine plumes. 



This is a rare species, and is described and figured 



by Montagu, Phil. Trans. Vol. xi. p. 17. Tab. iv. Fig. MoUuea. 

 4. ; probably the type of a new genus. % T""*' 



7. Doris /gn'crw Length about half an inch. 

 Cloak thickly covered with short lanceolate tubercles. 

 Above the head emarginate. Sheath of the superior 

 tentacula notched in the margin. Anal plumes about 

 eight in number. Body pale, frecked with dusky. 



\Ve have found this species in the Zetland seas, and 

 have not met with any description with which its cha- 

 racters correspond. 



II. PoLYCEn.\. This genus is nearly allied to th<8 p, 

 former. It was instituted by Cuvier, for the reception 

 of those species whose anus is surrounded with plumea, 

 but which possess, in addition, two membranaceous 

 plates wherewith to cover them. The superior tenta- 

 cula are similar to those of the Doris, but the inferior 

 ones, surrounding the mouth above, are more nume- 

 rous, extending from four to six. 



This genus contains fewer species than the preced- 

 ing. Two, however, have been detected as natives of 

 the British seas. 



1. Polycerajlava. Length about half an inch. Feel- 

 ers above the mouth four in number ; superior feelers 

 two, awl- shaped. Anal plumes seven, behind which are 

 two long smooth appendages. 



This species is described and figured by Montagu, 

 Linn. Trans, vol. vii. p. 79- tab. vii. fig. 6. The figure 

 exhibits two black dots behind the tentacula, probably 

 intended for eyes, although no notice is taken of them 

 in the description. 



2. Poli/cera pcnnigera. Length half an inch. Cloak 

 pointed behind ; contracted at the tentacula and emar- 

 ginate in front. Tentacula subclavated and psrfoliated, 

 with a sheath of two leaves. Anal plumes five, with 

 two bifid appendages. 



Described and figured by Montagu. Linn. Trans. 

 xi. p. 17- tab. iv. fig. 5. 



In those genera of this order, in which the anus is 

 situated on the right side, and unconnected with the 

 branchiae, some, as the Tritonia and Scyllea, are fur- 

 nished with corneous jaws, while in the thethys, there 

 are only muscular lips. The mouth in the other genera 

 has not been examined with sufficient attention. The 

 presence of jaws, however, is an important addition to 

 the digestive organs, enabling the animals to cut the 

 substances on which they feed, while the others must 

 swallow their food entire. 



III. TRITONIA. This genus was instituted by Cu. Tritonin. 

 vier, for the reception of those animals formerly ar- 

 ranged under the genus Doris, which have the branchiae 

 disposed on each side the cloak. Many of the species 

 possess eyes, or at least black points which are regard- 

 ed as such. There are two superior tentacula, as in the 

 Doris, which are in part retractile, and above the 

 mouth sometimes another pair, or in their stead plu- 

 mose appendages. 



The mouth consists of two lips, which are placed 

 longitudinally, and open into a short canal. Within 

 are the jaws, which consist of t\vo corneous plates unit- 

 ed at the upper dorsal edge, slightly arched, and meet- 

 ing at their upper margin for the purpose of cutting. 

 Within these is the tongue, which differs remarkably 

 from the same member in the Doris. In the latter, the 

 spines with which it is beset, are reflected, and draw 

 the food to the gullet, while in the former, the spines 

 are deflected, and serve to keep the food within the 

 reach of the jaws. The tongue of the Doris therefore 

 serves for deglutition, that of the Tritonia for mastica- 

 tion. M. Cuvier, by mistake, describes the functions of 



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