THE TREMATODA 



there is usually a small aperture on the dorsal surface which leads into a 

 narrow canal, known as " Laurer's canal," opening into some part of the 

 female duct system. There is never a separate vagina in the sense in 

 which the word is used in the Heterocotylea. 



JL 



FIG. IX. The Structure of a Schematic Malacocotylean. (Founded on that of a Distomum.) 



1. Plan of the alimentary canal and the nervous system, supposed to be seen from the 

 dorsal surface, o, the ventrally placed mouth (represented by dotted outline) lying in the 

 centre of the anterior sucker, also indicated by dots ; b, pharynx ; c, median portion of the 

 intestine (this may be long or so short as to be practically absent) ; d, the bifurcate intestine 

 which typically is without caeca ; e, the brain ; /, anterior nerve ; g, marginal (lateral) nerve ; 

 h, the ventral nerve ; i, the dorsal nerve stem (the commissures are not represented, see 

 Fig. XX.) ; j, the posterior, median excretory pore ; I, the dorsally placed aperture to which 

 Laurer's canal is seen passing upwards. 



2. Plan of the external anatomy of the ventral surfaces and of the excretory system, c, 

 posterior or ventral sucker ; d, the genital pore ; j, the excretory pore leading into a median 

 contractile bladder (k) which bifurcates anteriorly and receives on each side the duct or 

 collecting canal (0 ; this runs forward to a varying distance and then gives rise to two canals 

 (m) the anterior and (>i) the posterior canals ; eacli of which gives off branches terminating in 

 flame cells. 



3. Plan of the genital organs supposed to be viewed from the ventral surface, c, margin 

 of the ventral sucker ; d, genital pore leading into the genital atrium and receiving the male 

 and female ducts ; e, uterus, long and undulating ; /, ootype, surrounded by the shell gland ; 

 sr/germarium. The region between /and g is the germ duct, and receives on one side Laurer's 

 canal (1), and on the other the median vitello-duct (m) ; n, the vitellarium, with longitudinal 

 vitello-duct connected by transverse ducts to the median duct ; p, cirrus in its sac ; r, seminal 

 vesicle ; s, sperm duct, formed posteriorly by the union of the two testicular ducts ; t, the 

 testes, here compact and nearly always double. 



In the life-history of the Malacocotylea the fertilised egg gives rise to 

 a larva which, in order to complete the cycle, enters another ("inter- 

 mediate ") host ; here it usually gives rise asexually to numerous 

 individuals of a second form, and frequently these again to a third form, 

 from which the sexual worm is developed. So that from each egg 

 deposited by the adult a very large number of new flukes are developed. 



