SUB-KINGDOM VII. VERMES. 



THE WORMS. 



BY R. I. POCOCK. 



To give a concise diagnosis of the Vermes is a matter of no small difficulty, 

 for the different classes that are discussed in this chapter are merely grouped 

 together for the sake of convenience, since they present few features in com- 

 mon to justify their association, apart, that is, from the negative fact that 

 they do not possess the characters by which the rest of the sub-kingdoms 

 may be recognised. From the Echinoderms (star-fish, sea-urchins, etc.) and 

 Coelenterates (corals, jelly-fish), for example, they may be distinguished by 

 beng bi-laterally and not radially symmetrical; bi-laterally symmetrical 

 anmals being those in which the right and left halves of the body are alike. 

 From the Arthropoda (insects, spiders, etc.) they may be recognised by the 

 abisence of jointed limbs and of limbs modified to act as jaws ; from the 

 Chordata (Vertebrates and Ascidians) by the absence of the cartilaginous 

 doirsal chord (notochord) and of slits in the walls of the gullet ; and from the 

 Mollusca (snails, cuttle-fish, mussels) by the absence of the gland for pro- 

 ducing the shell, and of the muscular foot, which subserves locomotion. In 

 addition, however, to these negative features, it may be said that the body is 

 soft, usually long, and often jointed ; the alimentary canal, when present, 

 usually traverses it from end to end ; but in the lowest forms it has no pos- 

 terior outlet, and in some of the parasitic species has entirely disappeared. 

 In the simpler types the nervous system consists of two chords, one running 

 along each side of the body, and uniting in the head to form the brain, which 

 is placed just above the gullet. But in the higher groups these chords are 

 united throughout their length, and form a double chain, traversing the body 

 from end to end beneath the alimentary canal. In the higher forms, again, 

 there is a highly developed circulatory system, consisting of longitudinal and 

 transverse vessels, but no such organs have been observed in the less higTily 

 organised types. Very characteristic, too, of all the classes are the excretory 

 vessels, consisting of tubes that open to the exterior, and at their inner 

 end communicate with the body cavity. 



A bond of union, however, between some of the classes, which in the adult 

 stages are very dissimilar, may perhaps be sought in the similarity that is 

 observable in their larval forms. For example, in the groups known as 

 Polychreta, Gephyrea, Nemertinea, and Turbellaria, in addition to some 

 others of less importance and extent, the young is an active larva furnished 

 with cilia, or vibratile hairs, arranged in one or more definite belts around 

 the body, by the movement of which it is enabled to swim freely through the 

 water. These groups would thus seem to have at least a common starting- 

 point in development, from which they have diverged in different directions. 



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