Structure and Development of the Nephridia. 359 



vascular system (see Gamble & Ashworth, 1900, Fig. 26). The 

 second diaphvagm thus constitutes in reality the posterior boundary 

 of somite III, Avhile the tliird diaphragm similarly delimits somite IV. 

 This disposition of the diaphrag-ms is characteristic of ali the species 

 of Arenicola that have thus far been examined, and has its origiu 

 in A. cristota in a mauner that has already been sufficiently descri- 

 bed above. The first chaetigerous somite is therefore, strictly spea- 

 king, the second body-somite and not the third as Gamble & 

 AsHWOKTH suppose (1900). It is highly improbable that dififerences 

 in this respect should prevali among the dififerent species oi Areni- 

 cola; at ali events in A. cristata, study of the larvai stages proves 

 beyond a doubt tbat the composition of the anterior region is as 

 above described. 



The Position of the first nephridium in A. cristata (body somite 

 VI, or 5*^^ chaetigerous somite) agrees with that of the majority of 

 other species hitherto investigated. In A. niarina, however, the first 

 nephridium occupies somite V (4*^ chaetigerous), and its nephrostome 

 perforates the third diaphragm and opens into the body-cavity 

 beyond. It is noteworthy, as Gamble & Ashworth have emphasized, 

 that this nephridium is typically smaller than the others and fre- 

 quently much reduced or even absent. Since the larvae of A. cristata 

 possess pronephridia in both somites V and IV, it is evident that the 

 present condition has been reached through the disappearance of 

 certain more anteriorly situated nephridia; and the process of re- 

 duction is apparently stili in progress in A. marina. The forma- 

 tion of the anterior diaphragms is possibly in part responsible for 

 the disappearance of the nephridia of this region, since the nephridia 

 are thus cut off from the genital region and deprived of one of their 

 most characteristic functions, that of conveying the genital products 

 te the exterier. 



In anatomical structure the nephridia of -4. cristata are very 

 similar to those of A. marina. The large conspicuous nephrostome 

 with its fringed dorsal lip transversed by the nephrostomial blood- 

 vessel, the spacious and uncoiled glandulär region, and the large 

 contractile terminal vesicle are characters apparently possessed in 

 common by ali species of the genus. The nephrostomial blood- 

 vessel, after leaving the posterior margin of the dorsal lip, passes 

 along the body of the nephridium and is associated in body-somites 

 VII to X with the formation of the gonads. These organs have the 

 characteristic form of Strands of cellular tissue traversed by the 



Mittheilungen a. d. Zool. Station zu Neapel. Bd. 17. 24 



