The proscolex in T. serrata gives origin to a single scolex, and the resulting organism is therefore termed Cysticercus. When it produces a number of scolices, the resulting organism is a Coenurus, e. g. C. cerebralis of the Sheep, the cause of the disease known as 'sturdy,' 'gid,' or 'staggers;' and when scolices are produced not directly by the proscolex, but indirectly from 'brood-capsules,' which originate from the proscolex in the first instance and remain attached to it, the organism is an Echinococcus.

For Figure of Coenurus, see P1. xiv. fig. 5.

The cyst in which the Cysticercus lies is formed by the irritated tissues of its host. Its inner surface is covered by a layer of epithelioid cells: its walls are composed of connective tissue and contain blood-vessels. The whole structure is produced, apparently, by the metamorphosis of lymph-cells.

The life-history of T. serrata and of other Cestoda is generally supposed to include three successive generations: two asexual, the proscolex and the scolex; one sexual with numerous individuals, the proglottides. The last-named are supposed to be produced, one after another, by posterior gemmation of the scolex, from which they are detached in many instances either singly or in groups. Many interesting features of resemblance between a fully-formed proglottis and a Trematode have been pointed out by P. J. Van Beneden (cf. Vers Intestinaux, p. 251, and P1. xxvii). But the facts do not appear to necessitate the view that the proglottis is an individual: and Riehm especially has drawn attention to certain particulars. The setting free of a proglottis may be paralleled with the setting free of the hecto-cotylised or sexual arm of many male Cephalopoda: the formation of new proglottides with the re-development of this arm, or to the building up of a complete worm from two or three somites, as in the Oligochaete Lumbriculus variegatus. Considered as an organism, a proglottis is unable to maintain its own existence: it has no organs of adhesion, and as a rule it is placed by its detachment under destructive influences.

If however it remains under favouring circumstances, i.e. within the intestine, it may increase in size (Vers Intestinaux, p. 249; Vers Cestoides, pp. 123, 143), just as the fragments of a Nemertean may continue to live and mature sexual products. However, both Leuckart and Van Beneden are inclined to regard this fact as decisive for the zooid-nature of a proglottis. But it is not clear from Van Beneden's account to what the increase is due; nor perhaps is it absolutely certain that dissection of the host may not cause separation of the joints from the strobila. Such separation often occurs with the slightest disturbance (cf. Vers Cestoides, p. 139). Turning to anatomy; - though the deep longitudinal muscles do not extend across the interval between successive joints, and are interrupted even in Ligula, yet there are longitudinal muscles which do so (Riehm); the nervous system is continuous throughout the worm: so too the excretory system, but the transverse anastomoses of the Taeniae do not exist in other forms; the ovaries of Ligula appear to be continuous (Moniez), and discharge of the proscolices from the uterus may take place long before the joints are detached, e.g. in Bothrio-cephalidae and Ligulidae. Moreover the primitive terminal joints may remain barren or develope sexual products at a relatively late period.

The degree to which segmentation is marked externally is variable: and in Triaenophorus and Ligula is scarcely discernible, much less so in the latter than in the former. The formation of joints is usually held to depend on the evolution of the sexual organs. It is however well marked in the young Ligula, much less so in the sexual worm: so too Triaenophorus. It exists in the barren posterior region above-mentioned, but this may be considered as arrested in growth. The setting free of the joints may well be an adaptation, and is possibly due to the completed development of the embryoes and consequent regressive metamorphosis of the genitalia.

Other regressive changes appear to occur at the same period. Hamann states that the nerves degenerate in the ripe joints of T. lineata: and Megnin contends that the scolex may lose its hooks and its suckers, and may even atrophy away completely. See Journal de 1'Anat. et Physiol. xvii. 1881. Such changes may occur, according to him, in T. serrata and T. solium. Donnadieu found that Ligula sometimes undergoes digestion when its ripe ova are discharged. Facts such as these show that no absolute conclusion can be based on the growth of joints after their detachment in the tapeworms of some fish (supra).

As to the two supposed asexual generations, the proscolex and the scolex, the question appears to turn on the following points: the asexual reproduction of the proscolex as proscolex: the mode in which the scolex takes origin from the proscolex: the character of the connection between proscolex and scolex: and the apparent necessity for two hosts.

The proscolex does not generally multiply itself asexually, but gemmation takes place in the Staphylocystis of Villot, in Echinococcus (?), and in an Echino-coccoid form discovered by Metschnikoff (cf. Leuckart, 'Parasiten,' i. p. 464); and it is possible that an imperfect fission may sometimes occur, e. g. in Coenurus. Such increase may be compared with the fission of a Trematode Sporocyst, e. g. of Fasciola hepatica, an undoubted representative of a generation, or with its gemmation as in Leucochloridium: with the division of the embryo Lumbricus trapezoides, an immature individual: with the formation of germs by the tail, i.e. an organ, of the Cercaria-larva., Bucephalus polymorphus, or of Cercaria cristata, according to the observations of Ercolani: or with the separation of parts of the hydrocaulus or stem which develope into individuals in a Campanularian, the Schizocladium of Allmann. In other words, the occurrence of asexual reproduction does not necessarily prove that the proscolex is an individual.

The scolex is derived from a local thickening of a layer of cells, part of the body-wall of the proscolex: but its muscular layers and its excretory system become completely continuous with the corresponding structures of the proscolex. This mode of origin is utterly unlike the way in which germs originate from the walls of a Trematode Sporocyst or Redia, viz. by the growth and division of a cell from the layer lining the coelome, or from the mass filling it at first (cf. Thomas, Life History of the Liver Fluke, Q. J. M. xxiii. 1883, p. 125, and figs. referred to): nor does it resemble the mode in which buds are formed in the Metazoa with the participation of all the germinal layers. With reference to this last point, however, it must not be forgotten that the germinal layers are at no period distinct in the Cestoda. It may be added that in a form discovered by Gruber (Z. A. i. 1878) in Cyclops serrulatus, there is no apparent proscolex at all, and the scolex developes without invagination.

The proscolex and scolex may remain permanently attached to one another as in Archigetes Sieboldi. The connection may only be severed when the first joints are detached, as in many Phyllobothrians and Phyllacanthians which infest fish.

Or the scolex may separate from the proscolex, and either enter an intermediate host, e. g. some species of Tetrarhynchus, or even in rare instances remain free (cf. Moniez, Travaux, etc. iii. 1, p. 142). Finally, the proscolex may be digested in the stomach of the second host, but so also are the barren joints formed by the scolex of Cysticercus fasciolaris (= Taenia crassicollis) while still encapsuled in the Mouse. There is evidently much variety observable with reference to the character of the connection between the two structures.

It is a general fact that two hosts are necessary for the evolution of a Cestode. Archigetes however is an example of the sufficiency of a single host. Ligula attains immature sexuality in a fish, its first host, so that a few days' sojourn in its second host, a water-bird, brings about sexual maturity. Taenia solium may exist in the flesh of man in the Cysticercus-stage, in the alimentary canal as a Tapeworm; and it has been shown by Riehm and Leuckart that the scolex of C. pisiformis may de-velope into a jointed and sexless worm in the intestines of the Rabbit, but whether it would ever attain sexual maturity under these conditions is doubtful. That the proscolex may develope in an alimentary canal is proved by P. J. Van Beneden's discovery of proscolices with scolices in all stages of growth in the intestine of the Lump-fish (Cyclopterus). The same thing possibly occurs also with other Tapeworms inhabiting fish. Why there should be two hosts is a most obscure question. Leuckart appears to think that there is a physiological connection between the two, inasmuch as one is the prey of the other.

Moniez has broached the idea that a change from one to another host is, in these animals, which are so completely parasitic and therefore dependent, necessary to maintain their vigour.

The facts adduced certainly weaken the generally adopted view as to the existence of an Alternation of Generations in the Cestoda. There can be little doubt that the strobila is to be regarded as a single organism, and the same statement is probably true of the scolex and proscolex1. It is evident that the Cestoda are profoundly modified by parasitism, not only structurally but developmentally. The study of lower forms in the class may throw more light on the many obscure points connected with its evolution, but unfortunately there are great intrinsic difficulties in the way of such an investigation.

Development of scolex and life-histories. Leuckart, op. cit., Moniez, Les Cysticerques, and P. J. van Beneden, Vers Intestinaux and Vers Cestoides; see p. 228, ante.

Archigetes Sieboldi, Leuckart, Z.W. Z. xxx. (Suppl.), 1878; Gruber, Z. A. iv. 1881.

A Gemmating proscolex, Staphylocystis, Villot, Tenias des Musaraignes, A. Sc. N. (6), viii. 1879; Urocystis, Id. A. N. H. (5), vii. 1881.

On the question of Alternation of Generations in Cestoda, see Riehm, Zeitschr. f. d. ges. Naturw. (Giebel), 54, 1881, p. 590; Moniez, Les Cysticerques, op. cit. pp. 135-154; Leuckart, Die Parasiten (ed. 2), pp. 488-490; P. J. van Beneden, Vers Intestinaux, p. 242 et seqq. Comparison with Trematoda. Van Beneden, ibid.

p. 251 et seqq.

Parasitism. P. J. van Beneden, Animal Parasites and Messmates, Internat.

Series, xix. 1876.

1 Moniez regards the scolex simply as an organ of adhesion. To this view Niemiec opposes the character and degree of development attained by the central nervous system which is always lodged in it.