62 THE TREMATODA 



wide collecting canals of the excretory organs, is provided with 

 distinct layers of circular and longitudinal muscles an unusual 

 condition amongst the Trematoda. 



The retractile sense organs arranged at definite intervals along 

 the margin of the ventral disc are extremely peculiar ; they are 

 supplied by a special nerve traversing this margin. 



The body is divided into an upper and a lower portion by a 

 horizontal muscular partition (Fig. VII.), extending anteriorly 

 from the genital pore as far backwards as the end of the in- 

 testine ; in the dorsal portion are situated the intestine, the 

 terminal portions of the genital ducts, and the vitellaria ; whilst 

 in the ventral portion lie the gonads, the wide excretory canal, 

 and the lateral nerves. 



With regard to the reproductive organs, the single globular 

 testis is exceptional, retaining the ancestral form. Certain parts 

 of the female apparatus are difficult of interpretation ; the recepta- 

 culum mtelli or receptaculum seminis is now regarded as homologous 

 with " Laurer's canal," which has lost its external opening (Looss, 

 Goto). It is stated to contain yolk spheres, and is developed 

 by proliferations of epiblast cells from the dorsal surface (44). 

 Self -copulation has been directly observed by Voeltzkow, and no 

 doubt constantly occurs, as frequently only a single parasite is 

 found. 



The egg develops into a young embryo, which differs from the 

 adult, chiefly in the fact that the sucker is posteriorly placed 

 on the ventral surface, and is relatively small (Fig. VI. 5-8). 

 There is also an oral sucker. The posterior sucker gets carried 

 forwards by the growth of the hinder dorsal part of the body ; 

 and the gradual formation of the compartments has been observed. 



The mode of infection is unknown, since the embryos have 

 only been hatched artificially, and it is unknown whether eggs or 

 embryos leave the body, and how they arrive in the organs in 

 which they are parasitic. 



ORDER 3. Malacocotylea, Monticelli ( = Distomea, Leuck. = Mala- 

 cobothrii, Burm. = Digenea, v. Ben.). 



Endoparasitic Trematoda in which the suckers are never more than 

 two in number, viz. a circumoral sucker, #nd a second, somewhere on the 

 ventral surface ; the latter may, however, be absent. In addition to 

 these two typical suckers, accessory organs are developed to aid in 

 fixation, in the form of glandular papillae, or scattered spines ; but there 

 are never any booklets or skeletal pieces on the chief ventral sucker. 

 The two forks of the intestine are usually without caeca. The excretory 

 system always opens to the exterior by a median posterior pore which 

 leads into a median contractile sac. In addition to the pores of the 

 sperm duct, and uterus, which normally open through a common atrium, 



