TREMATODA. 647 



e.g. in Tristomum, the lobes being extremely well-developed in some 

 instances, e.g. in the genus Onchocotyle. This gland gives origin solely 

 to the germ by the segmentation of which the embryo is formed. The 

 vitellarium is a symmetrical gland right and left. It consists very com- 

 monly of a number, generally very great indeed, of grape-like follicles 

 connected to ducts which unite, and finally open into two longitudinal 

 canals, one right, the other left, connected by a cross canal, usually 

 situated just in front of the germarium. At the centre point of this 

 canal a single duct which, as a rule, commences with a dilatation serving 

 as a yolk reservoir, leads to the duct of the germarium. Instead of grape- 

 like follicles, the gland may consist of two tubes provided with short caeca, 

 e. g. Calceostoina, some Distomidae ; or of two sacs as in D. ventricosum ; 

 or of several saccules arranged in a star-like fashion as in D. rufo-viride, 

 D. globosum ; or even of a net-work as in Monostomum mutabile^ and 

 M. reticulare (P. J. van Beneden). A canal, the Laurer-Stieda canal or 

 vagina, which is double in Polystomum^ Calicotyle^ Pseudocotyle^ single in 

 all other instances where it has been recognised, opens externally in 

 various positions, dorsally in Distomidae and Microcotyle, laterally in 

 Epibdella, Axine, and Polystomum, ventrally in Pseudocotyle and Calicotyle, 

 and near the uterine aperture in Tristomum. Internally it opens, when 

 single, into the vitello-duct, or germ-duct, somewhere near the union of 

 these ducts ; when double into the lateral vitello-ducts in Polystomum^ 

 the united vitello- and germ-ducts in Pseudocotyle and Calicotyle. The 

 canal itself is sometimes dilated, or there is a dilatation near its inner 

 extremity in which spermatozoa are usually found 1 . Its function is 

 doubtful. It has been supposed (i) to serve for copulatory purposes ; 

 (2) to act as a safety-tube for the escape of over-abundant or altered 

 vitelline products and spermatozoa. Copulation has been actually 

 observed to take place by its means in Polystomum (Zeller) ; and its 

 structure in Axine and Microcotyle appears to favour that view. On the 

 other hand its calibre is said to be too small to admit of copulation in 

 some Distomidae (Poirier) and in Calicotyle. Both vitelline cells as well 

 as spermatozoa and even germs have been observed in it in the former. 

 The united vitello- and germ-ducts are surrounded- where they merge 

 into the oviduct by a set of unicellular glands which secrete the shell. 

 In the monogenetic Trematoda the portion of the oviduct into which the 

 shell-glands open is dilated, variously shaped, and endowed with peristallic 

 movement. To this portion the name ' Ootype ' was given by P. J. van 



1 A receptaculum seminis ( = inner vesicula seminalis auctt) may be present as (i) a dilatation 

 of the Laurer-Stieda canal ( Tristomum, D. davatutn) ; (2) a pear-shaped vesicle near the inner end 

 of the same (D. palliatum, &c.) ; (3) a dilatation of the germ-duct (Onchocotyle appendicnlatcf). In 

 many Distomidae the sperm is aggregated at the spot where the shell-glands open. The term ' outer 

 vesicula seminalis ' is often applied to the dilated region of the vas deferens. 



