36 THE TURBELLARIA 



condition is related to the size of the animal ; for in the smaller 

 forms of Rhabdocoelids these organs are compact. 



The male and female organs are separate throughout their 

 entire course, there being two genital pores (the occurrence of two 

 genital pores in marine Planarians was first noted by Mertens), 

 the female pore being invariably behind the male (except in 

 Cryptocoelides). This appears to be an ancestral character ; and 

 we have seen that it frequently occurs in the Rhabdocoelida. 

 Though these two genital pores are usually some distance apart, 

 they come to lie very closely together in Stylochus, and the region 

 around them is slightly depressed; as this depression becomes 

 deeper a common genital atrium is formed in Stylochoplana agilis 

 and in Discocoelis. 



The general arrangement of the male organs will be seen from 

 the diagram (Fig. XVII.) there are one or two peculiarities in 

 regard to the copulatory organ which may be referred to. In 

 Stylostomum we have a repetition of the condition obtaining in 

 Prorhynchus and Cyliiidrostoma, viz. the penis and its sac open 

 externally in common with the pharynx and its sac. 



The simplest case of duplication of penes is seen in some 

 species of Thysanozoon, in which there is a pair (first recognised 

 by Claparede) close behind the mouth (Fig. XVI. 2); whilst in 

 Anonymus virilis some twelve or more pairs form a row on each 

 side of the ventral surface ; nevertheless, there is no duplication 

 of the female apparatus (Fig. XV. 9). In Cryptocoelides there 

 may be two, four, or even six penes, which, however, lie one 

 behind the other in a common antrum. Lang has suggested 

 that the "penis," with its glands, is developed from a simple 

 group of glands, having originally no relations to the sperm 

 duct j Bergendal's observations on Polypostia (3) seem to confirm 

 this view. .Here there is a circle of some twenty penes around 

 the female pore, and each is traversed by a branch of the sperm 

 duct, but the more posteriorly placed structures which resemble 

 them in all other points are deprived of this duct ; they are merely 

 glandular. We have here, as it were, a passage from some 

 indifferent condition of glandular organ to a specialised condition 

 in which these glandular organs become related to sperm ducts, 

 and therewith take on a new function. 1 



The female gonad is an " ovary," there being no differentiation 

 to form a special yolk-producing region. The various lobes or 

 germ-producing follicles are no doubt connected with the oviduct 

 on each side, but such connection has not, in all cases, been traced. 

 The oviduct on each side, or uterus, as it is sometimes called, 



1 Some interesting and suggestive facts concerning the use of the penis will be 

 found in Whitman's paper "Spermatophores as a Means of Hypodermic Impregna- 

 tion," Journ. Afurph. iv. 1891, p. 386. 



