604 THE ANIMAL KINGDOM. 



The number of organs present appears to correspond with the number of 

 nephridia in many Tubicola. The testes and ovaries are usually very 

 similar, and probably always originate from peritoneal or sub-peritoneal 

 cells. They are scarcely visible save at the period of sexual activity, and 

 consist in Oligochaeta of a single mass ; in Polychaeta of a number of grape- 

 like masses in which the developing ova and spermatozoa are contained 

 within a membrane. Cell-limits are not always traceable in the young 

 organ *. The genital products appear to be set free into the coelome, and 

 to ripen there in Polychaeta and the Oligochaete Chaetogastridae . The 

 testicular cells of other Oligochaeta ripen in special pouches developed from 

 the septa the so-called vesiculae seminales paired in the terrestrial, un- 

 paired in the aquatic, forms. The ovarian cells ripen in a similar way 

 in some of the aquatic Oligochaeta, but in the terrestrial are detached from 

 the ovary only when mature. Special efferent ducts vasa deferentia and 

 oviducts are present in all Oligochaeta, and in Eudrilus the oviducts are 

 continuous with the ovaries, a feature not observed in any other Chaetopod 

 (Beddard). Accessory organs are also present ; special copulatory setae 

 which replace the ordinary setae ; a clitellum at once a copulatory organ, 

 and an organ for secreting the cocoon, produced at the sexual season by 

 the hypertrophy and increase in number of the cutaneous glands in certain 

 somites ; (one only or several) receptacula seminis which closely resemble 

 nephridial vesicles (p. 602, and note), and the vesiculae seminales above men- 

 tioned. Much variety exists in the disposition of these parts (pp. 205-8). 

 Though strong structural resemblances exist between the genital ducts and 

 nephridia in Oligochaeta, it is by no means certain that the ducts are 

 modified nephridia. As to Polychaeta, the escape of the genital products 

 by the nephridia has been observed in Hermella, Arenicola, Tercbella, and 

 it probably occurs in other instances as well. It may take place also by 

 rupture of the body-walls 2 . Accessory organs of generation are rare in 

 this order, so far as is known, and are (perhaps) to be regarded as modifica- 

 tions of nephridia or parts of nephridia 3 . 



1 The anterior somites of the hermaphrodite Microphthalmus (ffesionidai) are male, the 

 posterior female (Bobretzky, Z. A. iii. 1880, p. 139). So Protula. In Spirorbis communis it is the 

 reverse. Hesione sicula and Tyrrhena, on the contrary, have hermaphrodite organs, which take the 

 form of longish cylindrical processes, with a blood-vessel in the centre. The surrounding peritoneal 

 cells change into ova and spermatozoa commingled (Eisig, Mitth. Zool. Stat. Naples, ii. 1881, p. 298). 



8 Cosmovici found ova in the nephridial vesicles of Arenicola, and witnessed their escape by the 

 nephridiopores in Terebella and Hermella. A. G. Bourne states that the slightest irritation of the 

 body-walls of Polynoe clava, when tense with accumulated genital products, causes rupture and dis- 

 charge. But the products were never found in the nephridia. Kallenbach, however, states that they 

 escape by the nephridia in Polynoe cirrata. 



3 A pyriform vesicle is attached to the nephridia of the sixteenth and following somites in 

 Alciope, and is filled with sperm in male and female alike. It represents possibly a nephridial 

 vesicle. In the hermaphrodite Microphthalmus there is a pair of penial papillae between the second 

 and third setigerous somites. The male ducts open on the apices of these papillae. The ducts are long, 

 coiled, and furnished with ciliated funnels, and may be modified nephridia. The nephridia in the 



