ANNELIDA. 



535 



which have been removed, seems but little inferior to that of the 

 Hydra (§-301); a circumstance which is peculiarly remarkable, 

 when the much higher character of their organization is borne 

 in mind. They possess a distinct pair of nervous ganglia (//), 

 from which branches proceed to various parts of the body; and 

 in the neighborhood of these are usually to be observed a number 

 (varying from 2 to 40) of ocelli, or rudimentary eyes, each having 

 its refracting body or crystalline lens, its pigment layer, its 

 nerve-bulb, and its cornea-like bulging of the skin. The integu- 

 ment of many of these animals is fur- 

 nished with "thread-cells" or "filifer- 

 ous capsules," very much resembling 

 those of Zoophytes (§ 310). 



362. Annelida. — This class includes 

 all the higher kinds of worm-like ani- 

 mals, the greater part of which are 

 marine, though there are several spe- 

 cies which inhabit fresh water, and 

 some which live on land. The body 

 in this class is usually very long, and 

 nearly always presents a well-marked 

 segmental division, the segments being 

 for the most part similar and equal to 

 each other, except at the two extremi- 

 ties ; but in the lower forms, such as 

 the Leech and its allies, the segmental 

 division is very indistinctly seen, on 

 account of the general softness of the 

 integument. A large proportion ot 

 the marine Annelids have special re- 

 spiratory appendages, into which the 

 fluids of the body are sent for aeration ; 

 and these are situated upon the head 

 (Fig. 274), in those species which (like 

 the Serpula, Terebella, Sabellaria, &c.) 

 have their bodies enclosed by tubes, 

 either formed of a shelly substance 

 produced from their own surface, or 

 built up by the agglutination of grains 

 of sand, fragments of shell, &c. ; whilst 

 they are distributed along the two 

 sides of the body, in such as swim 

 freely through the water, or crawl over 

 the surfaces of rocks, as is the case 

 with the Nereidoe, or simply bury 

 themselves in the sand, as the Areni- 

 cola or "lob-worm." In these respi- 

 ratory appendages, the circulation of the fluids may be distinctly 

 seen by microscopic examination ; and these fluids are of two 



circulating apparatus of TirebeUa 

 conchHega : — o, ]^bial ring ; 6, b, tenta- 

 cula; c, first segment of the trunk; 

 d, skin of the back ; e, pharynx ; /, in- 

 testine; g, longitudinal muscles of tlie 

 inferior surface of the body ; ft, glan- 

 dular organ (liver 1) ; i. organs of gene- 

 ration ;i, feet ; fc, k, branchi£B ; 2, dorsal 

 vessel acting as a respiratory heart 

 m, dorso-intestinal vessel; m, venous 

 sinus surrounding cesophagus ; n', in- 

 ferior intestinal vessel; o, o, ventral 

 trunk ; pj lateral vascular branches. 



