G20 



MOLLUSCA. 



Mollusca. this fin near the neck. They consist each of a small fleshy 

 **^*~Y~^-' cone striated across, with a semicircular sheath behind. 

 The branchiae consist of a tapering fleshy stalk, spirally 

 twisted towards the summit with a series of filaments 

 on one side. They are fourteen in number on each 

 side, alternately and oppositely small and large. The 

 anus opens in front of the third branchia on the right 

 side. The orifice of generation is exhibited under the 

 first branchia of the same side. In front of each of 

 the larger branchiae M a small cavity with a small 

 filament in the centre. 



The mouth is situated underneath the tentacula. It 

 consists of a large funnel, covered within with soft pa- 

 pilla?, destitute of jaws or tongue. The gullet is short, 

 the stomach simple, fleshy, and covered with a thick 

 cuticle. The salivary glands are slender and branched, 

 and open into the gullet. The intestine is likewise 

 short, and proceeds directly to the anus. The liver 

 pours the bile into the canal at the pylorus; and like- 

 wise sends out another duct, which opens externally 

 near the anus. The organs of generation are similar 

 to the Doris. 



The Thethys fimbria is the only species of the ge- 

 nus which has been satisfactorily ascertained. 

 Scyllea. VI. SCYLLEA. The distinguishing characters of this 



genus depend on the two tentacula, the mouth being 

 furnished with jaws, and the back with membrana- 

 ceous expansions bearing on their upper surface the 

 branchial papillae. 



The foot is very narrow, forming a groove on the 

 under surface. The body is ovate and compressed la- 

 terally. There are two expansions or crests on each 

 side, and one at the tail. The branchiae are in the form of 

 plumes covering the upper surface of the crests and the 

 back. The tentacula are each contained within a large 

 funnel-shaped sheath. 



The mouth is placed at the base of the tentacula, 

 turrounded with a semicircular lip. The jaws are 

 homey, and cut by crossing each other. The tongue 

 is furnished with reflected points. The gullet is lon- 

 gitudinally plaited. The stomach is short and cylin- 

 drical, with a ring of hard longitudinal scales. The li- 

 ver consists of six unequal globules. The bile is pour- 

 ed into the cardiac extremity of the gullet. 



The Scyllea pelagica has been long known to na- 

 turalists, and appears to be very common in the equa- 

 torial seas. It is commonly found adhering to the stems 

 of the fucus natans. 



Claucus. VII. GLAUCUS. In this genus, whose true charac- 

 ters are still imperfectly explored, the head is furnished 

 with four simple tentacula. On each side the body are 

 three or four horizontal membranaceous expansions, 

 whose margins are fringed with the branchial filaments. 



This genus was instituted by R. Foster, and the spe- 

 cies with which naturalists have been longest acquaint- 

 ed, is the Doris radiata of Gmelin, which is figured in 

 the Phil. Trans, vol. Hi. tab. iii. It is not uncommon 

 in the Atlantic Ocean. 



It is probable that a more minute examination of the 

 animals of this order, than has hitherto taken place, 

 would lead to the formation of several tribes and fami- 

 lies. The number of the tentacula, the structure of the 

 mouth, the connection between the cloak and the foot, 

 the appendages of the anus, and the disposition of the 

 branchiae, lay claim to the notice of the systematical 

 zoologist. But the study of these animals can only be 

 carried on at the sea-shore, where they can be preserv- 

 ed alive in their native element. 

 5 



II. OIIDER, Cyclvlranchia. 



Mollusc], 



The animals of this order differ from the preceding Cjrbbran. 

 in the marginal distribution of the branchire. Be. '>'" 

 tween the foot and the cloak, or dorsal covering of the 

 body, the gills are placed, above the one, and beneath 

 the other. These are in the form of little leaves, which 

 constitute a fringed band on each side of the body ; in 

 some surrounding the whole body, in others interrupted 

 front or behind. Independently of this remarkable 

 position of the organs of respiration, M. Cuvier ar- 

 ranges the animals in which it occurs into two distinct 

 orders, and places them far apart in his system. It is 

 not easy to account for this distribution ; as the only 

 character of importance by which he has marked their 

 separation is derived from the circumstance that in the 

 animals of his first order, the sexes, though united, 

 require the mutual union of individuals, while, in the 

 other, each individual is supposed to be capable of im- 

 pregnating itself. In these last, however, the other 

 organs, of feeling, digestion, 01 protection, have no 

 common properties. Averse to any arbitrary arrange- 

 ments of this kind, we have here brought them toge- 

 ther under one order, and shall now proceed to consi- 

 der the characters of the genera. 



1. PHYLLIDIA. The body, in the animals of this Phyllida. 

 genus, is ovate. The foot is narrow in front. The 



cloak is broad, coriaceous, and destitute of a shell. 

 Towards its anterior extremity are two cavities, from 

 which issue the retractile superior tentacula, as in the 

 genus Doris. Nearly at the posterior extremity is ano- 

 ther cavity, containing the anus. This opening, though 

 similar in situation to that of the Doris, is merely a 

 short simple tube. The head is immediately above 

 the anterior margin of the foot, above which is the 

 mouth, having a small conical feeler on each side. 

 Under the margin of the cloak on the right side, and 

 about half way between the mouth and the middle of 

 the body, are two openings, in a tubercle, for the or- 

 gans of generation. The branchiae consist of slender 

 complicated leaves, which surround the body between 

 the foot and the cloak. The circle is interrupted at 

 the head and the tubercle of generation. 



The mouth is destitute of jaws. The gullet is sim- 

 ple, ending in a membranaceous stomach. The pylo- 

 rus is placed near the cardia, and the intestine goes 

 directly to the anus. The salivary glands are small, 

 and placed near the mouth. The liver is large in pro- 

 portion. 



The heart is placed in the middle of the back. The 

 auricle is simple, placed on the side next the tail, and 

 supplied by the two systemic veins which collect the 

 aerated blood from the branchiae on each side. There 

 is a simple aorta arising from the opposite side of the 

 heart. 



The organs of generation appear to be similar to 

 those of the preceding class ; but they have not as yet 

 been minutely examined. The existence of eyes is not 

 satisfactorily determined. 



The animals of this genus appear to be inhabitant 

 of the tropical seas. Cuvier has given descriptions 

 and figures of three species, which differ remarkably 

 from one another in the protuberances of the cloak. 



2. DIPHYLLIDA. This genus was formed by Cuvier Diphyllids. 

 in his Regne Animal, for the reception of an animal in 



the cabinet of M. Brugmans at Leyden. It is but im. 

 perfectly characterized. The cloak is pointed behind ; 

 the head semicircular, with a feeler, and small tuber- 



