94 OLIGOCHAETA 



that the sperm-sacs are to be regarded as a testis ; but in putting forward this somewhat belated 

 theory, the author has forgotten the development of the organs in question. However these are 

 the grounds upon which the identity of the sperm-sacs with testes is based. 



The testes as first described by Heeing were not always to be found in the situation where 

 he described them ; bodies identical in every particular with these testes — so much so that one 

 might have served as a model for the other— were found in the sperm-sacs (' Samenblasen- 

 anhange '). Neuland also points out that younger spermatogonia were found in the extremities of 

 the sperm-sacs than in the more proximal regions; the reverse ought to be the ease were the 

 developing sperm-cells derived entirely from the testes (of Heeing) ; the nearer to the testes the 

 less advanced ought to be the spermatogonia. Heeing has figured the mouth of the sperm-duct 

 as nearer to the testes than to the mouth of the sperm-sacs ; why therefore, asks Neuland, does 

 not the unripe sperm get into the latter? As is known this does not take place. Another matter 

 raised by Neuland is the immense amount of sperm produced— an amount too excessive to have 

 been produced only by the testes of Heeing. 



Even if it be true that germinal tissue is produced in the interior of the sperm-sacs — which as 

 yet wants confirmation — it is not clear how this proves the contention that the sperm-sacs are 

 testes ; in a loose sense of the word they might be called so ; but then so they might have been 

 before, for the median sperm-reservoirs contain the true testes; no doubt the whole structure may 

 be called a testis in the same sense that the testis of Astacus is so called, or even the testis of 

 the vertebrate which contains besides the germinal tissue other structures. 



§ 3- Egg-sacs \ 



In the majority of Oligocliaeta the ova when ripe or nearly ripe are transferi'ed 

 to certain sacs in which they remain for a time before being extruded from the 

 body; these sacs are enormously developed in all the aquatic Oligochaeta, but appear 

 to be rudimentary structures in most of the terrestrial forms. In Rhynchelmis, for 

 example, the egg-sacs extend back as far as to the fifty-fourth segment, or even in 

 more mature individuals to the sixty-seventh. In other aquatic families similar 

 sacs are to be found; they are comparatively as large in the Tubificidae and 

 Naidomorpha. 



Nasse has described their development in Tubifex: they originate as outgrowths 

 of the dissepiment xi/xii ; into these sacs the sperm-sacs are pushed as they 

 develop ; and thus in the mature worm we have two sacs one within the other, 

 the outer of which contains ova and the inner sperm; the same is also the 

 case with Rhynchelmis, where Vejdovsky (9) speaks of the egg-sacs and the 

 sperm-sacs lying within and enclosed by a common membrane; it should be 

 observed that these sacs are paired, lying one on each side of the intestine (see 

 Vejdovsky 9, PI. Ill, fig. i o.s., fig. 2). But the egg-sacs are not always paired; in 

 Stylaria proboscidea (cf. Taubeb 2, PL XIV, fig. i o.v.), for instance, and in other 



' Often called Receptacula ovorum. 



