M O L L U S C A. 



Aplytia. 



plates which inclose the body. Between these, in the 

 middle of the right side, the branchiae are placed. The 

 tentacula are two in number, with eyes at the base be- 

 hind. 



The cloak is strengthened in the middle above the 

 branchiae by a thin expanded subspiral shell. The neck 

 is short, and in some contracted, with the front emar- 

 gir.ate, exhibiting the commencement of the inferior 

 tentacula. The upper tentacula are tubular and cloven. 

 The gills occur at the edge of the dorsal plate. In 

 front of these are the orifices of the organs of genera- 

 tion, and immediately behind the gills the anus is si- 

 tuated. 



The mouth is furnished with a short retractile pro- 

 boscis. The tongue occupies both sides of the mouth, 

 and is covered with spines. The gullet is enlarged into 

 a kind of crop before it enters the stomach. This is 

 folded, and is divided by contractions into three parts. 

 The first has muscular walls of moderate thickness, with 

 a single longitudinal band. In the second the walls 

 are membranaceous, with longitudinal internal ridges. 

 The third stomach has thin and simple walls. The 

 gut is short. The salivary glands are situated at the 

 folds of the stomach, and empty their contents into the 

 mouth by two canals. The liver is placed on the sto- 

 mach, and empties itself into the lower part of the crop. 



The heart is nearly in the middle of the back. Its 

 auricle is on the right side at the base of the bran- 

 chia>, and the ventricle sends out at the opposite side 

 three arteries. 



M. Cuvier has figured and described the P. peronii 

 with its anatomical details. Two species likewise ap 

 pear to be known as natives of the British seas. 



1. Pleurobranchus plumula.Foot pointed behind. 

 Gills in the form of a compound plumose appendage. 

 The cloak contains the shell known to British writers 

 as the Bulla plumula of Montagu, and figured and de- 

 scribed in Testacea Britannica, p. 214. Vignette 2. 

 fig. 5. 



2. Pleurobranchiis membranaceus.'Foot rounded, 

 with an irregularly indented margin. Cloak covered 

 with conical pappilice. Gills a plumose appendage. 



This species was found by Montagu at Kingsbridge, 

 Devonshire, in 1809, arid is described and figured by 

 him in the Linn. Trans, vol. xi. p. 184. tab. xii. fig. 3. 4. 



We have been induced to refer the two species to 

 this genus as agreeing generally with the characters 

 assigned to it by Cuvier. There is, however, one par- 

 ticular in which they greatly differ. Cuvier states, 

 that the gills in his Pleurobranchi " sont attachees le 

 long du cote gauche (droit !) dans le sillon entre le 

 manteau et le pied, et representent tine serie de pyra- 

 mides devisees en feuillets triangulaires." Eegne Ant' 

 mal, iii. p. S96. His representation of the P. peronii, 

 in his Memoires, accords with the preceding descrip- 

 tion. But in the two British species, the gills appear 

 like a feather, free at one extremity. They may be 

 permitted to remain, therefore, in the genus Lamella- 

 ria, which Montagu instituted for their reception, un- 

 less we suppose that Cuvier, from inspecting only 

 shrivelled specimens preserved in spirits, overlooked 

 Ihe true structure of the gills, a conjecture we consider 

 as highly probable. 



1 1- AFLYSIA. Tentacula four, the eyes placed in 

 front of the two superior ones. Mouth longitudinal. 



The body of the Aplysia is ovate, acuminated be- 

 hind, and produced before to form a neck. The foot 

 is narrower than the body. In the middle of the back 

 is a corneous plate enclosed in a bag in the skin, and 



on each side, and behind, there is a fold by which this 

 part may be concealed. The head is slightly emargi- ^ 

 nated, with a feeler on each side. The superior feel- 

 ers are situated on the neck. In front of each of these 

 is a small black point or eye. 



The branchicc are situated underneath the dorsal 

 plate, on the right side, and exhibit a complicated plu- 

 mose ridge, capable of expansion beyond the edge of 

 the plate. The anus is situated immediately behind 

 the branchiae, and before them is the orifice of genera- 

 tion, from which proceeds a groove along the neck to 

 the inferior base of the fore-feeler, on the right side, 

 where there is an opening for the penis. 



Within the longitudinal lips are two smooth corne- 

 ous plates, the substitutes for jaws; the tongue is rough, 

 as in many of the other gasteropoda. The gullet is 

 short, and suddenly expands into a large subspiral 

 crop, with membranaceous walls. To this, a gizzard 

 with muscular walls succeeds, the interior of which is 

 armed with numerous pyramidal teeth, with irregular 

 summits, of a cartilaginous nature. The connection 

 between these teeth and the integuments is so slender, 

 that they are displaced by the application of the small- 

 est force. They, however, project so far into the 

 cavity, as to offer resistance to the progress of the food. 

 There is yet another stomach, armed on the one sid 

 with deflected, pointed cartilaginous teeth. At the 

 pyloric extremity are two membranaceous ridges, be- 

 tween which are biliary orifices, and the opening into 

 a long narrow ca?cum, with simple walls> which is 

 contained within the lever. The intestine is simple, 

 and after two turns ends in a rectum. 



The salivary glands are very long, and, as usual, 

 empty their contents into the pharynx. The liver is 

 divided into three portions by the folds of the intestine, 

 each of which consists of several lobes. The biliary 

 vessels are very large, and open at the mouth of the 

 caecum into the last stomach. The food of the Aply- 

 sia consists of sea- weeds and minute shells. 



The circulating organs are remarkable. On each 

 side the body, in the region of the dorsal plate, there 

 is a large vessel, which receives blood from different 

 parts of the body, and which likewise, by various 

 openings, has a free communication with the cavity of 

 the abdomen. In this respect there is a resemblance 

 to the spongy glandular bodies of the venae cavte of 

 the cephalopoda. These two vessels, or veiia: cmw, 

 unite posteriorly, and transmit their contents to the 

 gills. The aerated blood is now conveyed to an au 

 ricle, of large dimensions, and uncommonly thin walls, 

 situated beneath and towards the front of the dorsal 

 plate, and emptying its contents through a valve, into 

 the right side of the ventricle. The aorta> which is- 

 sues from the left and anterior side, divides into two 

 branches, the smallest of which proceeds to the liver 

 on the left. The larger branch is again divided, the 

 erna'.ler branch proceeding to the stomach. The larg- 

 est trunk that remains, before it leaves the pericar- 

 dium, has two singular bodies attached to it. consist- 

 ing of comparatively large vessels, opening from this 

 aortic branch. The use of these glands is unknown. 



The organs of generation likewise exhibit some re- 

 markable peculiarities. The ovarium is situated in the 

 posterior part of the abdomen. The oviduct is tortu- 

 ous in its course, passes along the surface of the testi- 

 cle, and after uniting with a clavate appendage, opens 

 into a common canal. The testicle is firm, apparently 

 homogeneous in its texture, of a yellow colour, with 

 spiral ridges on its surface. The vas deferens arises 



