86 THE TREMATODA 



traverses a papilla arising from the floor of the atrium. When in 

 use, this papilla is merely protruded through the genital pore. 

 The penis is formed of two parts : (a) a muscular, connective 

 tissue sheath, enveloped in a membrane ; and (b) distally a 

 chitinous armature, either in the form of a crown of booklets, 

 lying on the outer surface of the papilla, and projecting therefore 

 into the atrium and not into the sperm duct, or a tube. In a few 

 instances ( Udonella, Diplozoon, and some species of Distomum) there 

 is no penis or cirrus, and there is every reason to believe that 

 self-fertilisation occurs. 



The germarium is single, the germ duct or oviduct is short, 

 extending as far as the special dilated region known as " ootype," 

 into which the shell glands open ; into the oviduct there open the 

 vitelline ducts from the vitellarium, which is follicular or distinctly 

 lobed, except in Udonella and Cakeostoma, where it is compact. 

 In Diplozoon it is unpaired, in the rest paired ; it always lies dorsal 

 to intestine, against which it is closely placed. Beyond this point 

 the " oviduct " is known as " uterus," 1 and passes forwards, usually 

 in a more or less undulating course, to the atrium genitale, or in 

 rare instances to its own separate aperture. 



But in addition to these organs there are certain ducts, the 

 homologies of which have been much discussed (Fig. XXIL). 

 (1) In the Heterocotylea there is typically a paired, or in other 

 cases a single vagina, the opening of which varies in position in 

 different genera (cf. Figs. I, II., III., V.). It is usually ventral, but 

 in Hexacotyle dorsal ; the single vagina appears, in some cases at 

 least, to be derived from the fusion of two, for in Axine heterocerca 

 the single (dorsal) pore leads into two ducts (Goto). In Poly- 

 stomum each vagina opens through twenty or thirty small pores 

 situated on the " lateral swelling." At its internal end the vagina 

 (XXIL 3, k) communicates with the transverse vitelline ducts, and 

 in its course is sometimes dilated to form a " spermatheca " ; the 

 vagina is the female copulatory organ for the reception of the 

 penis ; its pore is the " copulatory pore." 



The uterine pore may, therefore, in opposition thereto, be 

 termed the " birth opening." 



(2) In the Heterocotylea there is also a narrow duct passing 

 from the oviduct, opposite the entrance of the vitello-duct, to the 

 right limb of the intestine. This " genito-intestinal canal" (/) 

 whose true relations were discovered by Ijima, and since con- 

 firmed by Goto and all who have examined the matter, was 

 originally called the "internal vas def erens " by v. Siebold, and 

 believed to be connected with the testis, close to which it passes ; 

 it was then looked upon as a means for direct, internal self-fertilisa- 



1 Goto finds cilia on the uterine epithelium of several genera of Heterocotylea 

 and in the vagina of Calicotyle. 



