SECOND DIVISION. 



INVERTEBRATE ANIMALS. 



THE Invertebrate series of Animals include the Molluscs, Crustaceans, Insects, Radiated and Infusorial Animals ; none of which, varying 

 as they do from each other in several most important particulars, have an internal spinal arrangement. 



CLASS V. M O L L U S C A. 



THIS very extensive division of animals are characterised by being invertebral, soft, inarticulated, acephalous, or furnished with a more 

 or less prominent head at their anterior part, most frequently having eyes or tentacula, or crowned at its summit with arms ; 

 mantle various ; sometimes with its edges free on the sides of the body, and sometimes having its lobes united so as to form a bag, 

 which partly envelopes the body ; branchiae various, rarely symmetrical ; circulation double, one particular, the other general or 

 simple ; heart unilocular, sometimes with two divided and widely-separated auricles ; no gangliar medullary cord, but scattered 

 and not numerous ganglia and various nerves ; body sometimes naked externally, and either destitute of solid parts within, or 

 covering a shell or hard bodies, sometimes furnished externally with a sheathing or enveloping shell. These animals have been 

 divided by naturalists into several Classes, Sub-classes, and Subordinate Groups. 



MOLLUSCS are distinguished from Insects by the entire absence of any 

 jointed disposition of their external covering, which, on the contrary, is 

 either soft, consisting of an external skin, lined internally with muscular 

 fibres, as in the Cuttle-fish and Slug, forming a perfect envelope to the whole 

 animal, and, as in the former, furnished with arms or claspers ; or the 

 covering is partially of this kind, more specially collected into one particular 

 part of the animal, and forming its foot or locomotive organ, as in the Snail, 

 and partially a membranous bag, in which the viscera are contained, and 

 thence called the visceral bag, which is protected from injury by the enclo- 

 sure of a shell, as in the just-mentioned animal. To this covering of the 

 viscera, membranous as in the Snail, and all the Molluscs covered with a 

 shell or shells, or with a leathery envelope like the Ascidue, or skinny, with 

 a muscular lining, as in the Cuttle-fish, the term Mantle, pallium, has been 

 generally but loosely applied, for it gives the same name to things very 

 different. Again, the term Mantle is equally applied to the membranous- 

 like (laps of skin which turn off from the body of a Mollusc like the Oyster, 

 and enclose its sides as the covers include a book, and to the collar by which 

 the visceral bag of the Snail is connected with its foot, and still more 

 strangely to the shield-shaped portion of skin which protects the heart and 

 respiratory organ of the Slug. It would certainly be better to distinguish 

 these very different parts into, first, the visceral bag enclosing the intestines, 

 which, in the Naked Molluscs, as the Cuttle-fish, Slug, and the like, consists 

 of skin and muscle, and in those which are contained within a single or 

 univalve shell, as in the Snail, is membranous ; secondly, the collar which 

 surrounds the junction of the visceral bag with the foot, as in the Snails, 

 and all Univalve Molluscs ; and, thirdly, the mantle or leaf-like reflections 

 of the membranous visceral bag, observed in the Oyster and all Bivalve 

 Molluscs ('. e. such as have a pair of shells), and which serve the double 

 purpose of connecting and producing partially, if not entirely, the shells, the 

 interior of which they overspread. 



SUB-CLASS CEPHALOPODA. HEAD-WALKERS. 



THIS class is generally held as the highest of the Molluscs, from its pre- 

 sumed approximation to the Vertebrate series, in its possession of some 

 internal cartilaginous masses, of which the principal is considered as a 

 rudimental brain-case or skull for the partial protection of the large nervous 

 ganglions supposed to be analogous, to a certain extent, with the brain of 

 vertebrate animals. The < 'i-phdopods are so named from having their limbs 

 or arms disposed around the head, pretty much like the petals of a flower 

 around its stamina. The arms, when expanded, stretch out in a radiated 

 form, and the junction of their roots produces a thick muscular ring or nip, 

 its area overspreads with a loose skin, in the centre of which is placed the 



aperture of the mouth, containing a pair of horny jaws, their shape nearly 

 resembling that of a parrot's beak. The head and arms of the Cephalopod, 

 in its ordinary crawling motions, rest immediately, and more or less com- 

 pletely, upon the bottom of the sea in which they live, whilst the body or 

 trunk, consisting of the bag which encloses the viscera, rises above them 

 like a tree-stem above its roots : hence they may be justly described as 

 walking upon their head, a fact necessary to be remembered in connection 

 with the detail of their anatomical characters. But this movement is nut 

 the only one they are able to perform, for they have also the power of dart- 

 ing themselves through the water, or swimming, though not in the ordinary 

 acceptation of the term ; this motion being effected by the sudden expulsion 

 of the water contained in the cavity enclosing the gills, which jerks the 

 animal backwards. 



Cephalopods are divisible into two sections, which have been named by 

 Owen, in reference to the number of gills, or branchice, with which they are 

 furnished ; hence those having four gills are called Tetrabranchiate, whilst 

 such as have but two are Dibranchiate. 



The Tetrabranchiate section are connected with the Gasteropods by the 

 enclosure of their whole visceral bag within the outermost chamber of their 

 shell, and by the strong connection of the animal itself to the shell by means 

 of a pair of powerful muscles, arising from the cartilage which Owen calls 

 the body of the skeleton. Of this section the Pearly Nautilus is an 

 example. 



The Dibranchiate section. All the Naked Cuttle-fish are examples of this 

 section. 



ORDER OCTOPODA (LEACH). HAVING EIGHT FEET. 



ILLUSTRATIVE EXAMPLES. 

 PLATE 1. 



ARGOSAUTA. Shell univalve, very thin, involute; the last turn very 

 large. A double tuberculated dorsal carina. 



The species of this genus were all confounded by Linnaeus, in his Argo- 

 nauta Argo (the Paper Nautilus). The animal, which inhabits this beautiful 

 shell, is so nearly allied to the genus Octopus, or eight-armed Cuttle, 

 ailord but few marks of distinction. (See OCTOPCS.) Two of the arms of 

 the Argonauts are furnished with a large expanded membrane, by in,.. 

 which the calcareous secretion is poured out for the enlargement or reparatii m 

 of the shell. The opinion that these, membranes, when expanded, served 

 the office of sails, enabling the animal to glide upon the surface of the 

 water in calm weather, is now exploded. The shell is of a thin pa py re- 

 appearance, white and semi-transparent. Its tonn is paitieularly 

 elegant, reseml >ling a kind of boat or vessel, and is marked throughout its 

 surface by numerous deep furrows. Few lie conceived 



