Classification of the Asconidge. 355 



the superficial flat epithelium secrete the monaxon spicules 

 with which the surface always bristles, and have given up or 

 have not acquired the contractile function. My histological 

 studies are not yet sufficiently advanced to enable rae to state 

 this as a general proposition for the whole group ; but in the 

 young specimens, bred from the larva of variahilis^ I found 

 very clearly that each cell of the flat epithelium secretes a 

 single monaxon spicule, and I believe the same to be true of 

 the adults also. In the histology the collar-cells have the 

 nucleus in the upper part of the cell close under the collar *. 

 The larva, so far as it has been observed, is an amphiblastulaf, 

 the dermal cells not being covered by the gastral layer, and 

 the first spicules to appear in the development are monaxons, 

 so that the sponge passes through what may be termed a 

 " Protascyssa " stage. 



Finally, there remains for consideration the interesting 

 Ascandra falcata, H. This Ascon is remarkable for the fact 

 that the interior of the gastral cavity is thrown into folds, 

 forming radial tubes like a Sycon, though it should be noted 

 that these diverticula are not foldings of the whole body-wall 

 of the sponge, as in a Sycon, but of the inner layer alone . 

 The colony has the form of a loose reticulum of tubes, from 

 which arise at intervals the chimney-like oscula, whose 

 diameter is scarcely greater than that of the tubes. Thus in 

 form it more approaches the coriacea rather than the hotryoides 

 group, and in all other important points of structure, histology, 

 and development it also belongs to the former group. The 

 triradiate systems are equiangular, there is a well-marked 

 contractile epithelium distinct from the skeletogenous layer, 

 the collar-cells have the nucleus at the base, the larva is a 

 parenchymella J, and the first spicules to appear are tri- 

 radiates. In fact, the only character which separates this 

 species at all sharply from the coriacea group is furnished by 

 tiie folds of the gastral epithelium. Admitting this to be a 

 cliaracter which is suflScient to permit of separating falcata 



* The collar-cells of individuals of this group are often to be found 

 remarkably shortened, almost flattened, so that the height of the cell is 

 scarcely greater than the diameter of the nucleus. In such cases it is, of 

 course, impossible to say if the nucleus is basal or terminal in position. 

 Where, however, the cell has its normal more or less columnar form, the 

 nucleus is always distinctly termiual. The figures given by Bidder of the 

 collar-cells of iSyam comjjressmn &c. serve admirably to illustrate this 

 point (Quart. Journ. Micr. Sci. n. s. vol. xxxviii. pi. ii.). 



t Seen in Lieherkilhnii (Keller) and variabilis. 



X 1 found the larva at Banyuls-sur-Mer in July of this year. It 

 closely resembles the larvee of blanca and co7itorta, scarcely differing 

 from the latter except in size and shape. 



