M O L L U S C A. 



611 



task of doing to will be very difficult, as they are all 

 very minute. When it is considered that a gasteropo- 

 doas mollusca breathing air, Segmentina (Nautilus of 

 Lighfoot) palustris, has its shell divided into chambers 

 by transverse partitions, and that the shells of some of 

 the Annelides likewise exhibit appearances of being 

 multilocular, the analogy loses its value, and we are dis- 

 posed to conclude that the true place in the systemati- 

 cal arrangement of animals, for the introduction of these 

 minute shells remains to be ascertained. Indeed, until 

 the animals of the Spirula and Nautilus shall have un- 

 dergone a more minute investigation, the characters by 

 which they are distinguished must be viewed with sus- 

 picion. 



Many species of multilocular tcstacea occur in a fos- 

 sil state, which are usually considered as the remains of 

 ceph -lopodous mollusca. Some of these are spiral like 

 the Nautilus, others are orbicular like the Miliola, while 

 in a third the chambered shell is nearly straight, as in 

 the Orthoccra. 



At intimately related to the last mentioned genus, 

 we may here take notice of the btlemnitr. In this fossil 

 the apex is solid, with a groove or fold on one side, and 

 at the thick end there is a conical cavity filled with a 

 shell divided into chambers, which are penetrated by a 

 pipe, i f we regard this body as the remains of a ce- 

 phalopodous animal, we may consider the exterior -.olid 

 extremity to have been a corneous covering, and the 

 chambered alveolus as the teat of the body of the ani- 

 mal, which likewise enveloped the base. That the 

 olid base was bard, and not muscular, like the eac of 

 the Sepiacea, it obvious from the serpula- which have 

 been found adhering to itt surface, and which probably 

 took up their residence after the death of the animal, 

 and the destruction of the soft covering. That the 

 solid part was different in iu nature from shell, appears 

 probable from the circumstance that the latter, when 

 mineralized, is usually converted into calcareous spar, 

 while the former appears of a fibrous structure. 



The nlvroii of the belrumite bear to near a resem- 

 blance to the specie* of Orthocera, that tome hare con- 

 cluded that the Latter were originally parts of abeletunite. 

 Several circumstance*, however, militate against this 

 opinion. ( )rtliocertatit- are frequently found fossil 

 where there are no vestiges of belemnite.t, and even ap- 

 pear to occur in older rock*. M any recent species of the 

 genus Orthocera have been found on our own shores 

 without the vestige of an external covering. Had 

 they possessed any such solid apex, like the belemnite, 

 it is probable that it would have been detected in the re- 

 cent kinds, since it U sufficiently durable to retain its 

 form in the solid strata. The thell of the heleninite 

 was probably, in tome respect!, internal that of the 

 Orthocera was probably external, or covered only by the 

 common integuments. The views here given do not 

 greatly differ from those of Mr. Plat, in the Philotoplii- 

 cal Traiuacliuni, vol. liv. p. 38. 



II. CLASS. PTEROPODA. 



Thit class was instituted by Cuvier, for the reception 

 of a few genera, whose peculiar characters indicated the 

 impropriety of suffering tlum to remain in any of those 

 categories which had been previously established. All 

 the specie* arc small in siae, and the attempts which 

 have been hitherto made to invettigate their internal 

 structure, liave, in a great measure, failed in explaining 

 the function* of the organ* which are exhibited. The 



valuable papers of Cuvier, on the Clio, Pneumodermon Mollusc*. 

 and Hyalea, include nearly all the accurate informa- > ^"V" ' 

 tion on the subject of which naturalists are in posses- 

 sion. 



The general form of these animals is somewhat ovate. 

 The tunic appears in some genera, as the Clio and Pneu- 

 modermon, to be double, the external one soft and thin, 

 the internal exhibiting a fibrous structure, correspond- 

 ing to the muscular web of the skin of the higher 

 classes. In these animals, however, these two layers 

 are unconnected throughout the greater part of their 

 expansion. In some, as the cymbulia, the tunic is 

 cartilaginous, while in others it is strengthened by a 

 shell. In these last, the shell in the Limacina is a spiral 

 univalve, covering the abdominal viscera, and in the' Hy- 

 alea, where it serves the same purpose, it approaches in 

 character a bivalve shell. It is, however, destitute of a 

 hinge, the two valves being united together at their 

 caudal margins, and there is no appearance of a trans- 

 verse adductor muscle. 



The organs of motion in all the genera consist of two 

 fins, or membranaceous expansions, seated one on each 

 side the head. They have no foot wherewith to craw], 

 nor any sucker* by which they can adhere to objects. 

 They are therefore free animal*, moving about in the 

 water by means of their fins, and possessing, probably 

 at the same time, a power of varying their specific gra- 

 vity, as they are capable of varying, to a certain extent, 

 the form of their bodies, and of enlarging or reducing 

 their dimensions. There is nothing peculiar in their 

 nervous system. 



Their organs of digestion differ greatly from those of 

 the cephalopoda, which we have already considered. 

 'I hey are generally regarded as destitute of eyes and 

 ear*. Their tentacula are either seated on the head, 

 forming two complicated branches of filaments, or 

 spread along the margin of the tunic. There are no 

 arms for seizing the food. The mouth, however, is 

 furnished with lips, and, in some, an appearance of a 

 tongue at the entrance of the gullet. The salivary 

 glands are two in number, lengthened, descending a 

 considerable way into the abdomen, and pouring their 

 content*, by means of theirexcretory canals, into thecavity 

 of the mouth. The gullet, after being encircled by the 

 nervous collar, suffer* an enlargement, which has been 

 termed a crop, contiguous to wh-ch is the stomach. 

 Both these cavities exhibit muscular ridges on the inner 

 nirface. The liver surrounds the stomach, and is 

 intimately united with its contents, and pours in its 

 bile by numerous pores. The intestine is short, and 

 after making one or two turns, ascends and terminates 

 in the neck near the mouth. 



The circulating system in this class has been but very 

 imperfectly investigated. The pulmoiiic vessels are 

 unknown, but systemic veins, a single auricle, ventricle, 

 and aorta have been detected. The heart, in some, it 

 situated on the left, in others on the right side of the 

 body. 



The respiratory or aerating organs, exhibit very re- 

 markable differences. In the Clio they are in the form 

 of a fine net work on the surface of the fins ; in the 

 Pneumodermon they are conjectured to form leaf-like 

 ridges on the caudal extremity of the body, or if these 

 ridges are to be considered as particular kinds of fins, 

 the gills may be sought for on the membranaceous ex- 

 pansions of the neck. In the Hyalea the branchiu- form 

 a complex band on each side the body at the lateral 

 opening of the shell. 



The animals of this class are all hermaphrodite!. 

 5 



