354 MOLLUSCA. 



in the form of the respiratory organs, and, Hke them, live in the sea ; but they are hermaphro- 

 dites, hke the Nudibranchiata and Pulmonea. 



The Pleurobranchus, Cuv.* 

 The cloak and the foot both jut beyond the body, v.hich thus appears as if it were between two 

 bucklers. The former contains, in some species, a little oval calcareous plate ; in others, a horny one, 

 and in either case it is situated above the head. The branchia; are placed along the right side, in a 

 groove between the cloak and foot, and represent a series of pyramids divided into triangular leaflets. 

 The mouth, in the form of a small proboscis, is overhung with an emarginate lip, and with two tubular 

 cleft tentacula ; the orifices of generation are before, and the anus behind the branchiae. There are four 

 stomachs, of which the second is fleshy, sometimes armed vrith osseous pieces, and the third is garnished 

 with prominent longitudinal laminae. The intestine is short. 



Tliere are different species in the Mediterranean and Indian Ocean, some of which are large and beautiful. [We 

 have two British species.] 



The Pleurobranch^a, Meckel {Pleurobranchidium, Blainv.), — 

 Has the branchiae and the orifices of generation situated as in Pleurobranchus ; but the anus is above 

 the branchiae ; the margins of the cloak and of the foot project but a little, and upon the front of the 

 cloak are four short distant tentacula, forming a square which forces a comparison with the anterior 

 disk of the Aceres. I find but one stomach, with thin parietes, which is a mere dilatation of the 

 intestine. A greatly divided glandular organ opens outwardly behind the genital orifices. There is no 

 trace of a shell. 

 The only known species is from the Mediterranean. 



The Aplasia, Lin. 

 The margins of the foot are turned up into flexile crests, and, surrounding the back on every side, 

 they can be reflected over it. The head, supported on a neck of greater or less length, has the two 

 superior tentacula hollowed like the ears of a quadruped, and two others of a flattened shape at the end 

 of the inferior lip ; the eyes at the base of the former. Upon the back we find the branchiae in the 

 form of complicated leaflets, attached to a broad membranous pedicle, and concealed by a little cloak, 

 equally membranous, which contains a horny flat shell. The anus is behind the branchiae, and is often 

 concealed under the lateral crests : the vulva is to the right in front, and the penis issues from under 

 the right tentaculum. A groove, which extends from the vulva to the very extremity of the penis, 

 conducts the semen thither in copulation. A membranous crop, of enormous size, leads into a muscular 

 gizzard, armed inside with many cartilaginous and pyramidal bodies ; and this is followed by a third 

 stomach beset with sharp hooks, and a fourth in the form of a coecum. The intestine is voluminous. 

 These animals feed on sea-weed. A peculiar gland pours out, tlu-ough an orifice near the vulva, a 

 limpid humour, which is said to be very acrid in some species ; and from the edges of the cloak there 

 oozes in abundance a deep purple liquid, with which the animal discolours the water of the sea when it 

 perceives danger to be at hand. Their ova are laid in long glairy entangled filaments, as slender as 

 threads. 



There are found in our seas Apl. fasciata, Poiret, punctata, Cuv., and depilans, Linn. ; and the shores of foreign 

 countries possess several others. 



The Dolabella, Lam. — 

 Differs only from Aplysia in the position of the branchiae at the posterior extremity of the body, which 

 resembles a truncated cone. The lateral crest fits close to the branchial apparatus, leaving merely a 

 narrow groove. The shell is calcareous. 

 The species are found in the Mediterranean and in the Indian Ocean. 



The Notarchus, Cuv. — 

 lias the lateral crests united and covering the back, leaving merely a longitudinal fissure to conduct 

 water to the branchiae. These have no cloak to cover them, but in other respects they resemble the 

 branchiai of the Aplysia ; and the organization of the two genera is otherwise similar. In 



* The same as the Lamellaria of Montagu, [a name which the Botanists have usurped,] and the Berthellit ot Blaiiiville. [This genus, Pleu- 

 robranch^a. Umbrella, Spirtcella,and Siphonia, are placed in the preceding order by Rang.] 



