[Illustration: FIG. 98.—A, A larval _Echinorhynchus proteus_ Westrumb. further developed than in Fig. 97. Highly magnified. The entoblast has developed inside it the proboscis _a_; _b_, _b_, the giant nuclei of the ectoblast. B, The entoblast at a more advanced stage, the ectoblast is not shown. The outermost layer of cells will form the muscles of the body-wall; the body-cavity has appeared; _a_, proboscis; _b_, cerebral ganglion; _c_, body-cavity; _d_, _d_, the testes beginning to appear in the ligament; _e_, cells which will form the generative ducts.] Some of the details of the development are very remarkable, and a short account of them may be given. The segmentation of the egg is unequal; it results in the formation of a central biscuit-shaped mass of small cells and a peripheral mass of larger cells; the former is called by Hamann[218] the entoblast, the latter the ectoblast. From the entoblast arise all the organs of the body but the sub-cuticle and the associated lemnisci, which are formed from the ectoblast. The latter has a remarkable history; the cells begin to break down and lose their outlines, whilst their nuclei fuse together and form a small number of giant nuclei, which lie scattered throughout the syncytium thus formed. The syncytium surrounds the entoblast on all sides; by this time the anteriorly-placed hooks have appeared; in _E. proteus_ there {181}are ten of these, but the number is not the same in all species. The syncytium is in a fluid state, with a few gigantic nuclei floating in it; these now lose their spherical shape, and throwing out processes become amoeboid; in this way they bud off small portions of their substance, and from these the oval nuclei of the sub-cuticle and the lemnisci arise. The rest of the syncytium hardens into the fibrillar matrix of the sub-cuticle, leaving, however, scattered spaces which form the sub-cuticular sinuses of the adult. An interesting feature of _N. clavaeceps_ and _Arhynchus hemignathi_ is that the skin of the adult retains the larval features, and it and the lemnisci consist of a syncytium with a very few giant nuclei scattered through it. Hamann counted only eight in the skin and two in each lemniscus in the example figured on p. 178. [Illustration: FIG. 99.—A, The larva of _Echinorhynchus proteus_ from the body-cavity of _Phoxinus laevis_, with the proboscis retracted and the whole still enclosed in a capsule. B, A section through the same; _a_, the invaginated proboscis; _b_, proboscis sheath; _c_, beginning of the neck; _d_, lemniscus. Highly magnified. (Both from Hamann.)]