DEVELOPMENT WITHIN THE EGG. 



45 



contained in a groove on the lower surface of the male, into the veins 

 of the pelvis, where the eggs are deposited in masses. 



With respect to the stage of development 

 which the eggs have attained when they are 

 laid, the differences in various species are 

 considerable ; every stage, from the egg just 

 fertilised to that which contains a fully 

 developed embryo, is represented. According 

 to the length of time which the fertilised egg- 

 passes in the ovarian duct, it is either 

 unchanged, or has commenced to segment, or 

 may even contain a fully developed embryo ; 

 it happens sometimes, e.g., in Trichina spiralis, 

 that the embryos are hatched while in the 

 body of their mother, which thus becomes 

 viviparous instead of oviparous. It is not 

 uncommon to find all these different ways in 

 animals very closely allied, and it follows 

 therefore that the mode of giving birth to its 

 young affords no clue to the systematic 

 position of a parasite. Quite as varied also 

 is the subsequent history of their eggs; in FIG. 29.Distomum ha>- 

 some cases they remain for a long period matobium, male and female, 



, ., , , , , , . ,, the latter in the canalis 



almost until the young are hatched in the gynsecophorus of the former, 

 identical spot where they were deposited; 



while in other cases they are immediately extruded from the body 

 of their host, and undergo their further development at large . The 

 latter is the most usual, and may be taken for granted where circum- 

 stances favour the dispersion of the eggs. There are numerous 

 exceptions in individual instances, especially among the Epizoa, which 

 often deposit their eggs in a more or less elaborate manner upon 

 various processes of the body (lice, for instance, attach their eggs to 

 hairs ; Dadylogyrus, Diplozoon, &c., attach them to the branchiae of 

 their host). When in such cases the ordinary means of attachment 

 are not sufficient, the egg-shell is provided, as in the species just men- 

 tioned, with some special apparatus of attachment in the shape of 

 suckers or tendril-like processes. These structures are as important 

 to the eggs of parasites as the various similar structures already 

 alluded to (p. 6) are for the parasite itself. 



It very commonly happens among intestinal parasites that the 

 eggs are early extruded from the body of the host, since they are 

 continually being pressed onwards by the semi-fluid contents of the 

 intestine ; this is so often the case, that we are not acquainted with a 



