SKELETON IN THE INVERTEBRATA. l3 



and a thick internal one resembling the woody fibres of plants, but 

 of an animal nature, termed chitine and coccine, blended with por- 

 tions of phosphate of lime, magnesia and iron. These animals 

 also present distinct legs and wings. 



In the arachnida we meet with a more consolidated form of 

 skeleton; generally more than three pairs of legs : and, at the sides 

 of their head, a pair of sharp-pointed piercing" instruments, suited 

 to their retired, cunning, and carnivorous habits. These animals 

 throw off periodically their exterior coverings, like the larvae of 

 insects; and, like the Crustacea, they are capable of reproducing 

 their members when destroyed. The Crustacea affords us by far 

 the most solid form of skeleton met with in any of the articulata. 

 In the decapods it contains half its weight of carbonate of lime, 

 and a considerable proportion of phosphate, with traces of iron, 

 soda, and magnesia, all of which are secreted from the true skin. 

 These animals have generally five pairs of legs, two strong mandi- 

 bles, two pairs of slender maxillae, and two pairs of antennae. The 

 solid crust forming the skeleton of Crustacea is cast off periodically. 

 This is accomplished by the animal first detaching the cutis and 

 muscles from the inner surface of the old shell; "then secreting 

 from the surface of the cutis a new layer of epidermis ; next a layer 

 of colouring matter; and, within this, the calcareous materials of 

 the new shell. 



Mollusca, Cuvier ; Cyclo-gangliated animals, Grant. The 

 shells of these animals are formed of carbonate of lime, without 

 the phosphate, and are remarkable for their want of symmetry on 

 the two sides of the body, and their inconstancy in animals of simi- 

 lar structure. In the tunicata we meet with an exterior cartilagi- 

 nous skeleton, in some instances thick and opaque, as in the ascidia 

 — in others more delicate and transparent, as in the cynthia papil- 

 lata. The shells of the conchifera usually consist of two movable 

 pieces placed on the exterior of the body, connected by ligament 

 and muscle. All these animals, however, are not bivalved ; some, 

 as the pholades, have additional pieces at the hinge of the valve, 

 constituting the multivalves. They have a muscular foot, and a 

 pair of tentacula. The gasterpoda possess hollow, unilocular, co- 

 nical shells. Many of them have no shell, as the tritonia and 

 doris; some have a thin calcareous lamina within the skin of the 

 back, as the aplysia; whilst others have only a partial covering of 

 shell as the testacella. In the cephalopodous mollusca we recog- 

 nise the transition from the external unorganised shells of the in- 

 vertebrated tribes to the internal organised bones of the vertebrata. 

 The shells are sometimes external, as in the nautilus, and some- 

 times internal, as in the sepia. In this complicated class of ani- 

 mals we find a near approach to the cartilaginous fishes in the pre- 

 sence of cranium, spinal column, &c, in a mdimental form. 



