Fig. 97.—An egg of Echinorhynchus acus Rud. surrounded by three egg-shells. Highly magnified. The egg has segmented, and the cells are differentiated into a, the entoblast, and b, the ectoblast; c, spines. (From Hamann.)
Embryology.—After fertilisation the egg surrounds itself with several egg-shells, three of which are usually distinguished; the embryo is already far advanced in its development by the time it leaves the body of the mother and passes out into the alimentary canal of the Vertebrate host. It leaves the body of this second host with the faeces, and is eaten by the first or larval host, usually a small Crustacean or water-insect, but in some cases a fish, within whose alimentary canal it casts its membranes and becomes actively mobile. By means of a ring of hooks developed round the anterior end it bores its way through the wall of the alimentary canal, and after some time—three weeks in E. proteus—comes to rest in the body-cavity of its host. By this time most of the organs of the adult, with the exception of the reproductive glands, are already well established; the latter only attain maturity when the first host is eaten by the second, and the larvae find themselves in the intestine of a Vertebrate.
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 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].
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.)
The whole of the rest of the body is formed by the entoblast. Within the latter a circular split arises which separates a single layer of outermost cells from an axial strand of many cells (Fig. 98, B). The split is the future body-cavity; the axial strand forms the proboscis, its sheath, the cerebral ganglion, muscles, etc., and the ligament with the contained generative organs; the outermost layer of cells forms the muscular lining to the skin. It is interesting to note that these cells destined to become muscle-fibres are at first arranged as a single layer of cubical epithelial cells lining the body-cavity; most of them become circular muscle-fibres, but a few are pushed inwards so as to lie next the body-cavity, and these become the longitudinal fibres.
Classification.—Until recently the Acanthocephala were supposed to include but one genus, Echinorhynchus, with several hundred species, but Hamann[[219]] has pointed out that these species present differences which enabled him to divide the group into three families, each with a corresponding genus. To these I have ventured to add a fourth family, to include a remarkable species, Arhynchus hemignathi, described below. The characters of the first three families in the account given below are taken from Hamann's paper.