170 Annelida. 



several of the body segnieats. One pair (or more) of auditory 

 vesicles is present in some {e.g., Lugworm), in tte neighbourliood 

 of the cerebral ganglion. 



In one division of the Chsetopods^ gills of different f ormSj tufted, 

 pectinatCj or filiformj are present on certain of the segments, one pair 

 to each somite. They occur on the dorsal surface, at the bases of the 

 notopodia. In many Tubicolae {e.g., Serpula) the prostomial threads 

 also serve as gills. Most of the Chsetopods, however, possess no 

 special respiratory apparatus. 



The genital organs are very different in the Polychaeta 

 and Oligochseta, the two groups into which the Chsetopods are 

 divided. The former are almost always of separate sexes: ova 

 or spermatozoa are as a rule formed in a great number of segments 

 on the inner side of the body-wall, or on the septa, so that many 

 ovaries or testes are present, which, do not appear as well-defined 

 organs, but only as thickened spots in the wall ; the genital 

 products fall into the body-cavity, and pass out through the nephridia. 

 The Oligochseta are, on the other hand, hermaphrodite, and the 

 ovaries and testes, which are more definite organs, are present in 

 only a few segments, one pair in each; there is always only one 

 pair of ovaries, one or two of testes. The Oligochseta are 

 further distinguished by having special oviducts and vasa deferentia, 

 which, like the nephridia, open into the body-cavity by ciliated 

 funnels; but there are also nephridia in these segments, so that 

 these canals cannot be homologous with segmental organs. Instead 

 of an oviduct, there is, in some forms, only a pair of slits in the 

 body- wall. 



In the Eartt- worms (Fig. 134) the spermatozoa do not complete their develop- 

 ment in the testis, but the cells from which they are formed break loose, and are 

 received in a number of definite sacs (vesioulBe seminales) which are situated 

 just within the body- wall and open by pores into the body-cavity. Here they 

 develop into spermatozoa. In some forms there is a similar receptacle for 

 the ova. In the Earthworms (and other Oligochseta) there are, further, sacs 

 (spermathecse), which open on to the surface, not into the body-cavity, and 

 during reciprocal copulation, receive spermatozoa from the other animal. 



The nervous system, vascular system, and excre- 

 tory apparatus, have been referred to in the general account of 

 the Annelida. 



The development of the Polychseta is effected by a distinct 

 metamorphosis, which is not found in the Oligochseta. The 

 larvffi are free-swimming, and provided with cilia, which, in some 

 forms, extend evenly over the whole body; in others, constitute a 

 well-defined band on the often discoid anterior end, and frequently 

 a second ring at the posterior end ; or there may be a large number 

 of ciliated bands. The body of the larva is at first short, parapodia 

 are absent or present in small numbers ; it gradually attains a con- 

 siderable length, dividing into numerous segments provided with 



