220 Description of the Plates. 



to the looped portion^ with the anterior element of which 

 it is connected by the duct ; and in the segmental organs^ 

 which are in the relation mentioned with the testes^ the 

 vesicular portions occupy the aug-le bounded by the vasa 

 deferentia and the inner parts of the looped portions^ and 

 alternate in position with the testes. The coecal process 

 constituting" a fourth portion in the segmental organs in 

 relation with the nine testes, is always brought into relation 

 with the anterior one of the two testes, with which the 

 segmental organ alternates, except in the case of the last 

 testis, in which the coecal process appears only to reach the 

 vas deferens of that last gland. In this figure four seg- 

 mental organs are represented as existing posteriorly to the 

 last testis ; in nature there . are only three, the most ante- 

 riorly placed one of which is connected with the male 

 organs as just mentioned ; whilst the two posterior ones 

 have their coecal processes prolonged nearly up to the middle 

 ventral line occupied by the nerve-cord. The seven seg- 

 mental organs situated anteriorly to the most anterior 

 testis, are represented in this figure by four. None of these 

 seven possess the inwardly prolonged coecal process, but 

 consist of a simple loop-shaped portion, the outer and larger 

 end of which lies almost vertically in the natural condition, 

 whilst the inner and smaller is prolonged inwards, and 

 more or less horizontally, beneath the digestive tube, as 

 in the other segmental organs, a duct and a vesicle open 

 exteriorly. The coecal process when present may be taken 

 to be homologous with the open infundibulum of the ovi- 

 ducts, and vasa deferentia of the Lumbricidae (see pi. viii., 

 i 3, i 4, m). In some Hirudineae, the segmental organs may 

 resemble those of other Annelids, in opening by ciliated 

 infundibula into the general cavity of the body (Branchio- 

 bdella), or into the interior of one of the pseud-haemal canals 

 (Clepsine), but they never are subordinated, as in those orders, 

 to the function of conveying the generative products. In 

 the middle regions of the body, the segmental organs are 

 repeated at regular intervals, five annuli being interposed 

 between the outlets of each pair; and the annulus imme- 

 diately posterior to these outlets carries a nerve ganglion on 



