INFLUENCES OF DIGESTIVE JUICES. 59 



cases, corresponding to the chemical and physical qualities of the egg- 

 shell, the solvent power of the digestive juices must vary, in order to 

 set free the enclosed embryo. The differences in the digestive 

 activities of various animals are but slightly understood, and, in fact, 

 are merely known to exist ; but we cannot doubt that they exercise a 

 profound influence upon the presence and distribution of parasites, 

 when we remember that the eggs of the common tape-worm 

 are digested by mammals, but not by frogs. It would appear that, 

 on the whole, the digestive activity of cold-blooded is less than that of 

 warm-blooded animals, since the larvae of flies, wood-lice, millepedes, &c., 

 and the shells of the eggs of tape-worms and of Ascaris lumbricoides are 

 able to pass through the alimentary canal of the former without being 

 digested. Moreover, since, as we have seen, it depends upon the charac- 

 ter of the egg-shell whether the embryo of a parasite be hatched outside 

 the body of its host or not, we are right in assigning to the first group 

 eggs with thin, delicate shells ; and this is especially the case in the 

 Nematodes (Dochmius, Sclerostomum, &c.) But these thin-shelled 

 eggs do not afford so much protection to their contents as those with 

 a stouter shell ; they are not found, therefore, in all species with free 

 embryos, and are always absent in those cases where the time of in- 

 cubation is longer. In these cases the eggs are thick-shelled, but 

 provided at one end with a kind of lid, which yields to pressure from 

 within, and can be raised up by the embryo (Fig. 38). These are found 

 in the Distomidse, Botliriocephalus, and in many 

 ectoparasites, e.y., the louse. But it must not 

 be supposed that the absence of a lid of this 

 kind hinders in every case the exit of the 

 young ; it is quite possible that the embryos 

 are enabled, by the possession of head spines ^ IQ 3g _ E g of 

 or other similar structures, to bore their way rioccphaius, with opercu- 

 through the egg-shell, as do the young of many lum ; the one is em P^ 

 other animals ; and in the case of Gordius, among Entozoa, this 

 has been actually proved. It is also possible that a damp environ- 

 ment may help to soften the shell, and so facilitate the escape of the 

 embryo, as has been observed in many thread-worms. 



Be that, however, as it may, the main fact of interest to us is 

 that there are numerous parasites which lead a free existence whilst 

 young. The majority pass this stage in water, in localities that the 

 egg has reached, in a more or less direct way, before the escape of 

 their embryos. Sometimes they swim about by the help of a covering 

 of cilia (Bothriocephalus, Monostomum, and other Trematodes Figs. 39 

 and 40) or special appendages (fish-lice) ; sometimes they remain at 

 the bottom, and make their way into the mud. Other species, 



