4< MANUAL OP THE MOLLUSCA. 



3. The various tribes of insects, spiders, crabs, and worms, 

 have no internal skeleton ; but to compensate for it, their outer 

 integument is sufficiently hard to serve at once the purposes of 

 bones, and of a covering and defence. This external ai'mature, 

 like the bodies and limbs which it covers, is divided into seg- 

 ments or joints, which well distinguishes the members of this 

 group from the others. The propriety of arranging worms with 

 insects will be seen, if it be remembered, that even the butterfly 

 and bee commence existence in a very woim-like form. This 

 division of jointed animals bears the name of the articulata. 



4. The fourth part of the animal kingdom consists of the 

 coral-animals, star-fishes, sea-jellies, and those countless micro- 

 scopic beings which swarm in all waters. Whilst other animals 

 are bi-lateral, or have a right and left side, and organs arranged 

 in pairs, — these have their organs placed in a circle around the 

 mouth or axis of the body, and have hence obtained the appella- 

 tion of radlata. 



These groups illustrate successively the grand problems of 

 animal economy. The lower divisions exhibit the perfection- 

 izing of the functions of nutrition and reproduction ; the higher 

 groups present the most varied and complete development of 

 the senses, locomotive powers, and instincts. We may also trace 

 in them an ideal progression from the simplest to the most 

 complicated structm-e and conditions. Commencing with the 

 Infusorial monad, we may ascend in imagination by a succession 

 of closely allied forms, to the sea-urchin and holothuria* ; and 

 thence by the lowest organized worms, upwards to the flying 

 insect. Or, starting at the same point, we may pass from the 

 polypes to the tunicaries ; and from the higher kinds of shell- 

 fish to the true fishes, and so on to those classes whose physical 

 organization is most nearly identical with our own. 



The mollusca are thus related to two of the other primary 

 groups ; — by the affinity of their simpler forms to the zoophytes, 



* See the HistoiT of British Star-fis)u;s, by Professor E. Forbes. 



