264 E. W. MACBRIDE. 



observed, is the only one of the great " schizocoelic " spaces 

 which Hamann (8) has described in the body-wall which has 

 any real existence, the others being merely artefacts produced 

 by the process of decalcification, I have found one specimen 

 showing the first trace of a dermal branchia (figs. 96 and 97). 

 We see a slight thickening of the peritoneum, and above it the 

 peribranchial space. Fig. 96 shows that the latter is a diverti- 

 culum of the coelom. As I have only one section illustrating 

 this I do not speak with absolute certainty on the point ; but, 

 with this possible though very improbable exception, there is 

 no schizocoele whatsoever in Asterina gibbosa : all 

 spaces lined by epithelium are of coelomic origin. 



Histological Differentiation. 



The cells of the gut-wall have undergone some change since 

 the close of the metamorphosis. Specimens of the epithelium 

 from different regions are given in PI. XIX, figs. 129 — 132. 

 These are all taken from a young adult in which R equals -85 

 millimetre. The cells of the lateral walls of the stomach (i. e. the 

 adult cBsophagus) have become exceedingly long and narrow; 

 their outer ends are refracting and take a light yellow tone 

 with osmic acid (fig. 129). The cells of the aboral wall, oa 

 the contrary, have developed numerous gland cells filled with 

 globules ; interspersed amongst them are some very narrow 

 filamentous cells. Fig. 130 shows the spot marked x where the 

 stomach opens into the pyloric sac and the abrupt change 

 in the character of the epithelium. The pyloric sac is lined 

 by uniform columnar cells ; the nucleus is generally near the 

 base of the cell, and is never placed further up than the 

 middle, and the protoplasm is uniformly granular (fig. 131). 

 The cells lining the rectal caecum (fig. 132) are similar in form 

 but smaller, and the protoplasm is clearer, with the outer part 

 more refringent. It is at least a plausible suggestion that the 

 gland cells of the stomach secrete the poison which paralyses 

 the prey, and that the cells of the pyloric sac give rise to a 

 digestive ferment. 



The differentiation of tissues which has gone on in the 



