IV VEBMES—GENITAL ORGANS 259 



two ovaries. From each ovary an oviduct arises, which, joining 

 that of the other side, passes into a sac -like muscular terminal 

 division, the vagina {Gnathohdellidm), or opens outwardly direct 

 {RhynchohdelRclce). 



In the land leeches {Lumlricohdella, Cylicobdella) the oviducts remain separate 

 until they enter the vagina. The ovaries here occur in numbers as swellings of the 

 o-idducts. In the BhynchoMellidic the ovaries are elongated sacs ; in the aquatic 

 GnathobdellidcE they are short and lobed, and usually contained in a round sac. In 

 Birudo the ducts unite, some distance before entering the vagina, to form a common 

 oviduct, into which numerous glands (albumen glands) enter. The female aperture 

 of Hirudo lies in the 11th segment, between the 35th and 36th rings. 



The arrangement of the hermaphrodite genital apparatus of the 

 Eirudinea vividly recalls the conditions which prevail in the Turbellaria 

 {Polyclada and Tridada). 



Oligroehseta. — The sexual, but more especially the transmitting, 

 organs of the Oligochceta are so completely different from those of the 

 Hirudinea, that it is not possible at present to refer the two to one 

 common type. Testes and ovaries are always very few in number ; the 

 former occur in one or two pairs, the latter in a single pair. The former 

 always lie in front of the latter. The collective male and female genital 

 apparatus occupies a limited number of segments in the anterior part of 

 the body. The segments lying between the 9th and 14th generally form 

 the genital zone. Less frequently {Aphanoneura, Chcetogastridw) the 

 genital organs lie farther to the front. While in the Hirudinea, Nemertina, 

 and Nematoda the egg and sperm passages are, as in the Platodes, 

 direct canal-like continuations of the ovaries and testes, they are, in 

 the Oligochceta, separated from the first from the germ glands, and show 

 much agreement ^vith nephridia in their structure. They are there- 

 fore pretty generally regarded as nephridia which have assumed the 

 function of conducting the genital products out of the body. We must 

 not, nevertheless, forget that the sperm ducts and oviducts, even if 

 they really are transformed nephridia, must in the segments referred 

 to represent supernumerary nephridia, since, in the adult Lumbricidce 

 and in the young stages of other Oligochceta, besides the oviducts 

 and sperm ducts, typical nephridia occur in the genital segments 

 as well as in the rest of the body. In the male genital ap- 

 paratus we must distinguish three parts, viz. the testes, the sperm 

 sacs, and the sperm ducts. In the iMmhicidcc there are almost always 

 two pairs of testes, while the rest of the Oligochcuta only possess one 

 pair. The testes seem everywhere to break up at an early stage into 

 the formative cells of the spermatozoa, so that in adult sexually 

 mature animals they are retained at the best as rudiments. The 

 sperm formative cells are early gathered into special sperm saes, in 

 which they develop further and produce the ripe spermatozoa. These 

 sperm sacs, formerly regarded as testes, are large vesicles which 

 develop on the dissepiments of the testicle segments as sac -like 



