244 Elementary Biology. 



Here, however, both cavities are present. The relation of 

 the one to the other, and the structure of the body-walls, 

 will be best understood by the examination of a transverse 

 section of the middle of the body. 



Externally covering the entire surface, save where the 

 nephridial, alimentary, and reproductive apertures occur, lies 

 a very thin and tough membrane, not formed of cells, but of 

 a cuticular substance known as chitin, chemically related 

 to keratin (p. 27), This membrane or cuticle is easily re- 

 moved, and is probably secreted by the layer (ectoderm) 

 immediately beneath it. That layer is cellular and corre- 

 sponds to the outer or ectoderm layer of Hydra. The cells 

 forming it are columnar, and have scattered amongst them 

 many glandular cells, whose duty it is no doubt to secrete 

 the cuticle. The ectoderm covers the double muscular 

 layer. The outer circular muscles are arranged in the form 

 of an almost continuous sheet from one end of the body to 

 the other, but the inner longitudinal layer is broken up into 

 bands which are separated from each other by the rows of 

 bristles and the openings of the nephridia. The septa are 

 thin, partly muscular, partly fibrous plates originating from 

 and continuous with the body-wall. 



On the ventral surface of each somite four pairs of minute 

 pits occur, from each of which projects a long curved bristle, 

 or seta, thicker towards the middle and tapering to either 

 end. These bristles are chitinous in their nature, and are 

 of cuticular origin. The ectoderm retreats into and lines 

 each pit, whilst there are minute strands of muscle fibre 

 springing from the body-wall and attached to the sunk end 

 of the bristle, by means of which the seta can be projected 

 and withdrawn at will. 



The nervous system and nervation. In connection 



blastic, or endoderm cells ; whilst the coelom is a space between two 

 layers of mesoblast, containing organs derived, it may be, from all 

 three layers. In animals like Hydra, in which the mssoderm is 

 wanting, manifestly no coelom can occur. 



