To this type the name metacoele might be applied 2. (3) It is an enterocoele, i. e. the persistent cavity of diverticula of the archenteron (supra), as in Amphioxus, Balanoglossus, Sagitta, Brachiopoda, and Echino-dermata. There remain the coelomic cavities of Vertebrata, the isolated nephridial pouches and genital ducts of Peripatus, the series of cavities in segmented Vermes, the pericardium, nephridial, and perhaps genital cavities of Mollusca. In these instances they are now usually regarded as enterocoeles abbreviated in development, a view which has most probability so far as the Vertebrata are concerned, and if a distinctive name is applied to them, crypt-enterocoele might be suggested; or they may be simply splits in the mesoblast, not derived from enterocoeles, and the well-known term schizccocle may be retained for them. Whatever value is attached to the coelome, the result of its presence is in most instances a division of the mesoblast into two portions, one applied to the body-wall, the other to the mesenteron, for which when the separation takes place in the embryo, the names somato- and splanchno-pleure are used.
It remains only to state that the epiblast gives origin in the adult to the epi- or hypo-dermis, to exo-skeletal structures, to glands, to organs of special sense and the nervous system, to the stomo- and procto-daeum and organs derived from them; the hypoblast to the intestinal or mesenteric epithelium, to the epithelia of glands or other outgrowths derived from the mesenteron, such as the lungs, thyroid, thymus, and the notochord of the Chordata. Reproduction by fission, or by gemmation in which the three layers are always (?) implicated, is not common; but the power of reproducing lost parts is met with in Coelcmata as high in the scale as Lacertilia1.
1 See on the primitive streak, Sedgwick, Q. J. M. xxiv. p. 79, and xxvii. p. 530; Haddon, Introduction to the Study of Embryology, 1887, p. 41; and the various references given in the Index vol. ii. of Balfour's Comparative Embryology.
2 Sedgwick uses the term pseudocode, but it would apply equally well to an archicoele, or to a schizocoele, if there is such a thing, supposing his definition of a coelome to be accepted (Q. J. M. xxvii. p. 533).
The connection between the Coelenterata and Coelomata is probably only that of descent from a common form of ancestor, unless it be supposed that the larval Coelomates with enterocoelic diverticula have sprung from Gastrulae common to them and the Anthozoa.
The classes of Coelenterata as they now exist are specialised; it is possible that two phyla may be indicated, one which has given origin to the Porifera, the other to the three remaining classes. Among the Coelomata are found groups, the relations of which are absolutely uncertain, e. g. Brachiopoda, Polyzoa. Certain phyla, or lines of common descent, may be indicated in other cases with confidence. These are (1) the Chordata with which Balanoglossus and Cephalodiscus are allied, if they are not actually to be considered as Chordates; and (2) the Echino-dermata, both of which are related to ancestors with enterocoelic pouches; (3) the Mollusca, descended from a trochosphere-ancestor common to them and most Vermes; (4) the Arthropoda, segmented animals which probably have two separate lines of descent, one including Arachnida and Crustacea, with an ancestor represented possibly by a Nauplius-form; the other Insecta, and Myriapoda with Peripatus, the latter indicating perhaps an ancestry related to the segmented Vermes. There remain (5) the majority of Vermes, an assemblage of apparently very diverse forms; the remarks on pp. 583-4, and the enumeration of classes, pp. 585-6, may suggest possible connections.
1 For views on the various points touched on in the foregoing account, see Sedgwick, 'On the origin of Metameric segmentation,' etc, Q. J. M. xxiv. 1884; Id. 'The Development of the Cape Species of Peripatus) op. cit. xxvii. 1886, pp. 515-40; Caldwell, 'Blastopore, Mesoderm, Metameric Segmentation,' Q. J. M. xxv. 1885; Hubrecht, 'The relation of the Nemertea to the Verte-brata,' Q. J. M. xxvii. 1886. On the Mesoblast see also, Kleinenberg, 'Die Entstehung des Annelids aus der Larve der Lopadorhynchus, Z. W. Z. xliv. 1886; Hubrecht, on Linens, Q. J. M. xxvi. 1886; Salensky, on Pilidium, Z. W. Z. xliii. 1886; Metschnikoff, on the wandering cells of Asterids and Echinids, Z. W. Z. xlii. 1885, p. 656.
The task of unravelling the phylogeny of the subdivisions of Metazoa is one beset with extreme difficulty. The records of Geology have established such points as the pedigree of the Horse, the derivation of Birds from extinct Reptilians; they show the extreme antiquity of some living types, e. g. Insectans, Scorpions, Elasmobranchs in Silurian strata, the great prevalence in ages past of forms now extinct or almost extinct, their replacement in other instances by derived types. Other phenomena speak to vast changes: the present geographical distribution of many terrestrial and aquatic animals; mimicry, migration, social habits; the degeneration which has so evidently befallen certain types, due to a sedentary mode of existence, to minute size, to adaptation for a parasitic life, i. e. one dependent on the living tissues or vital processes of another animal or plant for sustained nurture, whether it be external or internal, ecto- or endo-parasitism; Alternation of Generations, first discovered by Chamisso in Salpa, and by Steenstrup in Hydroids, Trematodes, etc, whether in the form known as metagenesis, i. e. the alternation of asexual and sexual individuals, or as heterogamy, i. e. the alternation of parthenogenetic and sexual races, or in one instance (Angiostomum) of an hermaphrodite and self-impregnating individual with bisexual individuals; the occurrence of prolonged metamorphoses, such as are seen in many Arthropoda, and the shortened metamorphoses of the early ontogeny of most animals; the degradation of an individual into an organ - a rare occurrence - exemplified in the avicularia and vibracula of Polyzoa, or the converse phenomenon of parts of an individual becoming elevated into the semblance of a number of individuals, the most probable interpretation to be put on the strobila of Cestoda. Nor has natural selection left untouched the record written on the pages of the life-history of any animal; it has falsified it in various ways at every stage - the ovum, its segmentation, the embryo.