ENTOZOA. 91 



scribed as distinct species, and that the Holostoma cuticola, Nordmann, 

 is the pupa, and Diplostoma clavatum, the larva of the same species. 



In the Planarice the ordinary development is by ova ; and it 

 is remarkable to watch the instinct of these animals during ovi- 

 position ; although they have no limbs, they take the ova as they 

 are excluded, but covered with a gelatinous fluid, which they draw- 

 out into threads, and thus fix the ova to the stems of little aquatic 

 plants. All this is done by the flexible and extensile mouth 

 and neck. The ova are large, and contain numerous germ-masses, 

 which manifest remarkable peristaltic movements of the cell-mem- 

 brane surrounding them. After a time these movements cease, and 

 the germ-masses conjugate, two or more losing their defining mem- 

 brane and blending their albuminous and granular contents together. 

 The embryo is developed from these larger, conjugated germ-masses, 

 and acquires its ciliated skin and a mouth by which it imbibes the 

 surrounding yolk-matter before exclusion. Several embryos are 

 thus developed in a single ovum, but not in a constant number. When 

 there are few, the embryos are large ; when many, they are small, at 

 the time of hatching. The species that propagate by spontaneous 

 fission are the sxaaXler Rhabdoctsli, which show no trace of generative 

 organs, and which are, perhaps, larvae of some larger Plauari*. 



Of the order Acanthocephala which includes the most noxious of 

 the internal parasites, no species is known to infest the human body. 

 They resemble the neraatoid Entozoa in outward form, and in the 

 distinction of the sexes, but in their digestive system they still mani- 

 fest the sterelminthic type. 



The species of this order constitute but one genus, Echinorhynchus., 

 characterised by a more or less elongated, round, subelastic body, the 

 head having a retractile proboscis armed with recurved spines. 

 Similar spines beset the whole outer surface in the Echinorhynchus 

 hystrix. The Echinorhynchi abound in the lower animals, and are, 

 some cylindrical, and others sacciform. The largest known species 

 {^Echinorhynchus gigas) infests the intestines of the hog. As regards 

 the tegumentary and muscular system, it resembles the nematoid 

 worms, as well as in its dioecious generation ; but its digestive system 

 is very different, and somewhat obscurely developed. The proboscis 

 is provided with a sheath, which projects freely into the abdominal 

 cavity ; two long and slender muscles are detached from the inner 

 surface of the general muscular investment and are inserted into the 

 base of the sheath, and two muscles pass from the most anterior part 

 of the body backwards to the sides of the sheath, for its extrusion. 



At the base of the proboscis there is a group of ganglion-corpuscles, 

 from which a number of nervous filaments radiate, and perforate the 



