516 ARTHUR DENDY. 



posed bundles of large spicules (fig. 3, I. sp.) } situate in the 

 inner portion of the ectosome. Outside of these comes a broad 

 zone of irregularly interlacing, separate, large spicules, which 

 occupy the greater portion of the ectosome, and which give 

 support to the numerous bundles or tufts of small spicules 

 (fig. 3, sp. b.) which project outwards at the surface of the 

 Sponge. The only remaining portion of the skeleton is a zone 

 of somewhat obliquely disposed and not very well-defined 

 bundles of small spicules (fig. 3, i. sp.) occurring in the outer- 

 most portion of the choanosome. These bundles are separated 

 from the well-defined longitudinal bundles occurring in the 

 inner portion of the ectosome by the two layers of fibrous 

 tissue, which I shall describe later on. The remainder of the 

 choanosome appears to be entirely free from spicules, a fact 

 which is probably to be accounted for by its dense, cork-like 

 consistence rendering their presence unnecessary. 



The Spicules. — The spicules are all tylostyli of various 

 sizes, each consisting of a long, straight, or slightly curved, 

 cylindrical shaft, terminating at the base in a subglobular 

 head, and at the apex in a fine point. They are of two chief 

 sizes. The larger ones (figs. 4 and 5) are straight, or nearly 

 so, and measure up to about 09 mm. in length, with a diameter 

 in the thickest portion of the shaft of about 0*014 mm. The 

 small spicules (fig. 6), which are almost confined to the surface 

 brushes and to the innermost zone of longitudinal bundles, 

 differ somewhat in shape from the large ones. They are very 

 slender, frequently somewhat curved, and they taper very 

 gradually to almost imperceptible fineness at the apex. They 

 measure about 0*2 mm. in length by 0"002 mm. in maximum 

 diameter of the shaft. 



The Ectosome. — In describing the ectosome and choano- 

 some, I shall, of course, leave out of account the spicular 

 elements, which have been more conveniently treated of apart. 

 The ectosome (fig. 3, ect.) is composed for the most part of 

 a transparent, gelatinous-looking tissue, containing ill-defined 

 fibres and other mesodermal cells. Towards the outside it 

 becomes more granular and stains more deeply, and no doubt 



