224 THE ANIMAL PARASITES OF MAN 



their attachment on the skin or the gills or other organs of their 

 hosts, eggs which attach themselves by means of their filaments. 

 The embryonic development thus takes place outside the parent. 

 This also holds good for the eggs of many endoparasitic species, 

 although as a rule in these the eggs are always retained for a 

 longer time in the uterus. Moreover, they usually here undergo a 

 part or a whole of their development, and are eventually deposited 

 in those organs in which the adult forms are parasitic, but this is 

 not always the case, as the egg, e.g., of F. hepatica appears in bile (and 

 faeces) quite unchanged. By the natural passages they eventually get 

 out of the body, and in cases where such do not exist, as in the case 

 of the blood-vessels, the eggs pass out by means of the kidneys. 



(4) The embryonic development, after irregular segmentation of the 

 ovum into a number of blastomeres, leads to the formation of a solid 

 blastosphere or morula, which is surrounded by a cellular investing 

 membrane (yolk envelope), while the principal mass of the cells forms 

 the embryo, which uses for its nourishment the yolk cells, which have 

 in the meantime disintegrated (cf. footnote, p. 223). Usually, after 

 the ova have reached water the embryos hatch out, leaving the yolk 

 envelope in the egg-shell ; in other cases, however, the embryos only 

 hatch out after having been subjected to the influence of the intestinal 

 juices, that is to say, in the intestine of an intermediate host which has 

 ingested with its food the ova that have escaped from the primary host. 



(5) The post-embryonic development of the Trematodes is accom- 

 plished in various ways ; the process is the most simple in the ecto- 

 parasitic species (Monogenea), the young of which should certainly 

 be regarded as larvae, because they possess characteristics (cilia, 

 simple gut, etc.) that are lacking in the adult worms, but which, 

 nevertheless, pass into the adult state direct after a relatively simple 

 metamorphosis. In the Holostomata, 1 a group found chiefly in the 

 intestine of aquatic birds, and which rarely occur in other vertebrates, 

 the ova develop in water. The young are ciliated all over, and, after 

 having entered an intermediate host (leeches, molluscs, arthropods, 

 amphibians, fishes) living in the water, they undergo a metamor- 

 phosis into a second larval stage ; they then encyst and await trans- 

 mission into the final host, where they become adult Metastatic 

 trematodes, i.e., trematodes without asexually produced generations 

 (p. 229). 



In the remaining so-called digenetic trematodes (p. 230) one or two 

 asexual generations interpose between the miracidium and terminal 

 stage, so that quite a number of adult worms may originate from 

 one egg. Usually the young, which are termed MIRACIDIA 2 (fig. 129), 



1 Holostomata : Prostomata with (in addition to the oral and ventral suckers) a third 

 fixation apparatus, generally on a separate part of the body. 



2 [Also known as ciliated embryos. F. V. T.] 



