xxx GENERAL INTRODUCTION. 



coeles abbreviated in development, a view which has most probability so 

 far as the Vertebrate 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 Lacertilia *. 



The connection between the Coelenterata and Coelomata is probably 

 only that of descent from a common form of ancestor, unless it be sup- 

 posed 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 Coelo- 

 mata 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 (i) 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 



1 For views on the various points touched on in the foregoing account, see Sedgwick, ' On the 

 origin of Metameric segmentation,' &c., Q. J. M. xxiv. 1884; Id. 'The Development of the Cape 

 Species of Peripattis] op. cit. xxvii. 1886, pp. 515-40; Caldwell, 'Blastopore, Mesoderm, Meta- 

 meric 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 Lopadorhynchiis, Z. W. Z. xliv. 1886 ; Hubrecht, on Lineus, 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. 



