20 ARTHUR DBNDY. 



fact that the spicules in question are somewhat smaller than 

 the more ordinary quadriradiates. 



The Gastral Cavity and Osculum. 



As already pointed 5ut in my description of the external 

 form of the sponge, the gastral cavity and osculum are greatly 

 modified by the peculiar mode of growth of the sponge. The 

 gastral cavity, instead of being narrow and tubular, has become 

 wide and basin-like, and at the same time, owing to the con- 

 volutions of its wall, extremely irregular. The osculum has 

 thus become wider than any other part of the gastral cavity — 

 a condition the opposite of that which obtains in other Sycon 

 sponges. 



Sometimes traces of the gastral cavity may be found in the 

 stalk even of adult sponges, causing the latter to be more or 

 less hollow. This indicates that the gastral cavity originally 

 extended all through the sponge — a fact which is proved, as I 

 shall show later on, by the ontogeny. 



(e) The histology of the soft tissues. 



The terms ectosome and choanosome, proposed by Sollas 



(16) and adopted by myself (2) in describing siliceous and 



horny sponges, are not convenient for at any rate the great 



majority of the Calcarea, and it is better to classify the tissues 



simply under the heads ectoderm, mesoderm, and endoderm. 



I must follow the example of Schulze (14) in considering that 



the ectoderm of the larval sponge (in the case of the Sycons, at 



any rate) furnishes not only the epithelium of the dermal 



surface, but also the epithelial lining of the inhalant canal 



system ; while the endoderm lines the remainder of the canal 



system from the prosopyles to the margin of the osculum, 



and the mesoderm furnishes all the remainder of the sponge 



body. 



The Ectoderm. 



The ectoderm resembles exactly what Schulze has described 



(11) in Sycandra raphanus, consisting of a single layer of 



flat, polygonal epithelial cells lining the dermal surface of the 



sponge and the inhalant canal system. These cells are most 



