478 



ZOOLOGY 



SECT. 



d.s 



v.s n.c ov 



I.Hirudo 



d.v 



funnels, or may be more or less degenerate, as in Hirudo. In the 

 Rhynehobdellid Pontobdella a very interesting modification of the 

 nephridial system occurs. Instead of distinct nephridia, there is 



found on the ventral 

 surface of the body a 

 very complex network 

 (Fig. 382, wp/z.), which 

 sends off on each side 

 of each segment a 

 short branch termin- 

 ating in a nephro- 

 stome (list.), and a 

 similar branch which 

 opens externally (np.). 

 This arrangement re- 

 minds us of the ex- 

 cretory system of cer- 

 tain Earthworms and 

 of Turbellaria. 



The nervous sys- 

 tem always closely 

 resembles that of 

 Hirudo, as also do the 

 sense-organs. The 

 number of eyes is sub- 

 ject to considerable 

 variation : they may 

 be developed on the 

 posterior sucker, or 

 may be absent alto- 

 gether. 



Reproductive 

 Organs. The testcs 

 usually have the seg- 

 mental arrangement 

 found in Hirudo, their 

 number varying from 

 five to twelve pairs. 

 But in Nephelis they 

 are very numerous, 

 and are not arranged 

 segmentally. In the 

 Rhynchobdellida the 



muscular penis is absent, its place being taken by an eversible 

 sac or Imrsa copulatrix. The form of the ovary with its 

 containing sac in Hirudo is exceptional. As a rule, there is 

 an elongated hollow ovary, producing ova from its epithelial 



nsl 



2.Pon|-obdella 



nst 



3.Nef)he Ms 



PIG. 381. Transverse sections of three Leeches ; diagram- 

 matic, c. coslomic spaces ; a: crop ; <l. s. dorsal sinus ; 

 </. r. dorsal vessel ; 1. s. lateral sinus ; I. r. lateral vessel ; 

 n. c. nerve-cord ; neph. nephridium ; nst. nephrostonie ; 

 on. ovary; f.<. testis ; v. s. ventral sinus; r. r. ventral 

 vessel. (After Bourne.) 



