xxviii GENERAL INTRODUCTION. 



digestive function (Ctenophora). There appears between the ectoderm and 

 endoderm a gelatinous lamella the mesoglaea, which may be structure- 

 less, partly fibrillate, or invaded by cells derived from one of the two 

 epithelia. Sensory and nervous cells are epithelial or sub-epithelial ; the 

 same is true of the contractile cells, which may, however, become im- 

 bedded in the mesoglaea. The generative products are sub-epithelial and 

 localised. But a differentiation of the mesoglaeal cells may occur, princi- 

 pally in Porifera. Reproduction by division of the organism is rare ; by 

 buds or outgrowths of the ecto- and endoderm jointly, common ; by 

 the division of a single mesoglaeal cell, or the growth of mesoglaeal cells 

 plus endoderm cells confined to Porifera. See pp. 713-6 ; 804. 



The typical characters of the Coelomata contrast with those of the 

 Coelenterata as follows. The fundamental symmetry of the Gastrula and 

 the vertical axis passing through the blastopore do not persist. As a 

 rule equal right and left sides are distinguishable ; but the anterior and 

 posterior parts of the body, if the permanent mouth, as is most natural, is 

 taken as a point of reference, are not equal, the former being relatively 

 small, and constituting a more or less distinct head. The archenteron, 

 the mesenteron of the adult, communicates with ,the exterior by a 

 mouth, and as a rule by an anus. The blastopore may become obliterated 

 in its centre, and its two ends may coincide with the position of the future 

 mouth and anus as in Peripatus ; it may close from behind forwards, or 

 vice versa, and then the mouth or anus correspond respectively to the part 

 left open ; it may close and leave no trace ; or it may never be found at 

 all as in Insecta. The permanent mouth and anus of the adult are gene- 

 rally, perhaps always, formed by a more or less pronounced ingrowth of 

 ectoderm, either at the open part of the blastopore, or independent of it. 

 In the latter case it is a question how far the ingrowth coincides with the 

 obliterated part of the blastopore, or the spot where it might be expected 

 to be. To the oral and anal ingrowths, the terms stomodaeum and proc- 

 todaeum are applied. Instead of a mesoglaea, there is a cellular meso- 

 blast or mesoderm. Its cells in all cases lie or come to lie between the 

 epi- and hypoblast. They are formed at an early period in the ontogeny, 

 and as a matter of fact are derived in several ways the mutual connection 

 of which is disputed. They may have a single or a double source. As to 

 the former, the cells arise (i) as immigrants (mesenchyme cells), from the 

 walls of the blastosphere, or from its endodermal pole as in Nemertea ; 

 (2) from the walls of the archenteron close to the blastopore, e. g. many 

 Crustacea ; (3) from cells specialised at an early period at the blastopore, 



