COELENTERATA. 713 



larva, and persists throughout life, holding the position of the absent meso- 

 blast or intermediate cell layer of Coelomata. This mesoglaea varies in 

 amount and consistency : it is homogeneous, sometimes partially fibrillate, 

 and may be invaded by cells derived from the ecto- or endo-derm, or from 

 both. The cells of the ectoderm may be uni- or multi-laminar. They may 

 be differentiated into epithelium or covering cells, epithelio-muscle cells, 

 muscle cells, pigment cells, gland cells, cnidoblasts, skeletogenous cells, 

 sense with supporting cells, ganglion cells, and genital cells. The endo- 

 derm is unilaminar, rarely multilaminar and then, as a rule, only in limited 

 spots. Its cells may be differentiated, like the cells of the ectoderm, with 

 the addition of skeletal cells, but skeletogenous cells, ganglion cells, and 

 sense cells, with the exception of otolith cells, are rarely met with. Both 

 ecto- and endo-derm are primitively unilaminar in the larva. 



A persistence of the vertical axis passing through the Gastrula mouth 

 as the long axis of the body, and a symmetry of form round this axis are 

 typical of Coelenterata. When bilateral symmetry is established, or a 

 lateral elongation of the body in a vertical plane takes place, an even 

 balance with reference to the vertical axis just named is, as a rule, main- 

 tained 1 . Irregular growth, however, occurs in most Sponges. 



There are four classes, Ctenophora, Anthozoa, Hydrozoa, and For if era. 

 The three first named possess peculiar offensive structures, either adhesive 

 cells or cnidoblasts containing nematocysts, the former in the Ctenophora 



porting lamella of Coelenterata, for the following reasons : (i) It appears typically as a clear layer 

 between the epi- and hypo-blast of the embryo some time after their differentiation ; (2) not only 

 does it contain no cells at first, but may remain permanently in this condition ; (3) when cells enter it, 

 they never form a defined layer either sub-ectodermic or sub-endodermic ; (4) it is frequently 

 specialised in density and fibrillation ; (5) though its cells, when present, may assume particular 

 functions, the supporting lamina as a whole never gives rise to tissues in the same way as does the 

 mesoblast. Apparent exceptions are (i) the early developed group of cells said by Metschnikoff to 

 be derived from the endoderm in embryo Ctenophores, the source of mesoglaeal cells later on (see 

 note 2, p. 720) ; and (2) the cellular mass filling the planula of certain Sponges. But according to 

 Chun, mesoglaeal cells are derived in Ctenophores throughout life from the superficial and stomachal 

 ectoderm ; and the cells of the Sponge larvae in question appear to be the common source of both 

 endoderm and mesoglaeal cells. 



A gelatinous substance fills the blastocoele of Echinoderm larvae, the Nemertean Pilidittm, etc. ; 

 but it appears before the epi- and hypo-blast are differentiated, and is, therefore, probably not 

 homologous with a supporting lamella. Cells enter it either from the walls of the blastula or of the 

 archenteron. They subsequently give rise to continuous cell-layers, and the jelly disappears at the 

 same time. The cells referred to are therefore destined to a development never attained in any 

 Coelenterate. 



It is not likely that Coelomate forms are derived from Coelenterate. The latter are specialised 

 from a simple gastrula type, from which the Coelomata have also sprung, but in a different 

 direction. The great complexity often acquired by the Coelenterate ecto- and endo-derm points to 

 the same conclusion. 



1 The converse of these three propositions appears to be true of the Coelomata. The principal 

 axis of the Gastrula never persists as the principal axis of the body. Bilateral symmetry is always 

 established, but may be disguised or lost ; it is balanced with reference to the Gastrula axis, but this 

 is not the case with the elongation of the body. Cp. p. 584, on the Trochosphere, &c. For points 

 connected with the mesoblast and coelome, see General Introduction. 



