STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 217 



ingrowth of the ectoderm from the region of the osculum, 

 replacing and, as it were, pushing back the collared cells. I 

 do not, however, consider that there is any reason for supposing 

 this to be the case. In the case of the Homocoela I have 

 shown (1) that the lining of the gastral cavity is not always 

 composed entirely of collared cells, but that the apical rays of 

 the quadriradiate spicules which frequently project into this 

 cavity are clothed with a layer of flattened epithelium. I 

 suggested at the time that this epithelium might possibly be 

 derived from the mesoderm; but, on the other hand, it might 

 with equal justice be regarded as indicating a transformation 

 of collared cells into permanent cells, and we are quite at 

 liberty to suppose that a similar transformation, only on a 

 much larger scale, has taken place in the gastral cavity and 

 exhalant canals of the Heterocoela. 



The epithelium which lines the gastral cavity and exhalant 

 canals resembles minutely the ectodermal epithelium which 

 clothes the outer surface of the sponge and lines the inhalant 

 canal system. This will be evident from a comparison of figs. 

 59 and 60, representing respectively the epithelium from an 

 inhalant and from an exhalant canal of Sycon Rams ay i, the 

 differences which exist between the epithelium in these two 

 situations being obviously very slight and insignificant. 

 Similarly fig. 64 represents the epithelium from an exhalant 

 canal of Vosmaeropsis Wilsoni, which, if we allow for the 

 contraction of the cells due to difference in the method of 

 preparation, is practically identical with the ectodermal 

 epithelium from the same sponge represented in fig. 63. In- 

 deed, unless specially prepared, as with osmic acid, the 

 ectodermal epithelium, as already pointed out, exhibits a 

 precisely similar contraction, and in both cases this is such a 

 constant and well-marked feature that I am inclined to think 

 that it betokens a normal contractility during life. The 

 appearance of the endodermal pavement epithelium, as seen in 

 section, is shown in figs. 27, 28, and 61. The swelling in the 

 centre of the cell, where the nucleus is situated, is fre- 

 quently very strongly marked, and causes the epithelium to 



