38 An Introduction to Zoology. 



external shells ; the luuscles thus attached are however merely confined to 

 the operation of closing those shells which the elastic ligament joining the 

 hinge would else keep expanded. In the spiral univalves a consistency is 

 imparted to the greater part of the animal, closely attached by its integu- 

 ments to the convolutions of the solid testaceous envelope. In the annelida, 

 (earthworms, &c.) we have a series of horny rings, forming a kind of ex- 

 ternal skeleton ; in the insects and Crustacea, the horny integuments in- 

 vesting each segment, exhibit the most perfect form of this external 

 skeleton ; the hard sheaths covering the distinct joints of the ex- 

 tremities, the legs, &c. are very analogous in form, position, and various 

 modes of articulation, to the corresponding parts of the vertebral skeleton, 

 and afford an equally perfect mechanical system of levers. The same end 

 is to be answered, and the manner in which many different modifications 

 of structure are made to accomplish one and the same end, is among the 

 most striking proofs of the inexhaustible riches of creative providence. 



In passing from the crustaceous to vertebral animals, we might at first 

 be tempted to suppose that we found a connecting link in animals like the 

 tortoise, where the osseous skeleton is still principally external, the ribs, 

 &c. being connected with expanded integumental plates. But here again, 

 if we compare general functions and structure, we shall find nothing like 

 gradual transition, but instead of it an abrupt leap over a vast intermediate 

 chasm. In the vertebrata, indeed, we see the system of an internal 

 skeleton developed, from its very first rudiments in the sucking fishes, the 

 myxine and lamprey, in the former of which we have the first rudiment of 

 a vertebral column in a simple cartilaginous tube, with scarcely any trace 

 of division into separate vertebral rings. Along the upper surface of this, 

 the spinal nervous chords proceed, protected only by a thin membrane, the 

 whole being more analagous to the cartilaginous lamina found in the cuttle 

 fish, than to any other animal type. In the lamprey, this cartilaginous tube 

 becomes distinctly divided into vertebral rings ; in the sturgeon and other 

 cartilaginous fish, each ring gradually expands its inner margin into a thin 

 edge, leaving at first only a narrow central opening for the intervertebral 

 gelatinous fluid, and at length quite closing it, and dividing the intervertebral 

 spaces into perfectly distinct cells 5 and at the same time, by the develop- 

 ment of cartilaginous processes on the upper portion of each vertebra, 

 here affords a protected canal for the conveyance of the spinal marrow. 

 In the osseous fishes, these cartilages become perfectly ossified j and in the 

 reptiles and mammalia, the expansion of the central portion of the bodies 

 of the vertebrae nearly obliterates the intervertebral spaces, leaving room 

 only for the membranes which attach the vertebrae into a united column. 



propel themselves by rapidly closing the valves of their shells. The biphori, a race 

 of acephalous moUusca, without shells, by sucking in water at one extremity and 

 forcing it out at the other, move themselves rapidly forward on the principle of a 

 rocket. Worms crawl by the elongation and contraction of their annular segments. 

 The motions of insects and vertebrata are fully illustrated in the text. 



