230 B. W. PRIEST ON THE NATURAL HISTORY 



The Siliceous Sponges are very numerous, as every dredging ex- 

 pedition which has been organised fully testifies, always discovering 

 some new form or forms. It includes the Halichondria and others 

 of our shores, as also that beautiful Hexradiate group, comprising 

 the Euplectella or Yenus' Flower Basket, Rossella^ Holtenia and 

 others, specimens of which may be seen in our National Museum. 



The Keratose Sponges include the Chalina, Dt/sidea, and the 

 Sponges of commerce. 



Very few species can be made out by their external appearance ; 

 even Dr. Bowerbank himself could not, experienced as he was, always 

 determine the species without a microscopical examination. 



Most of the Calcareous Sponges, comprising the four British 

 Genera, viz., Grantia, Levcosolenia, Leuconia and Leucogypsia, 

 take a more or less definite form, and with little experience it can 

 almost always be told at once to what genus they belong. For in- 

 stance in Grantia compressa and G. ciliata we find the Sponge to be 

 constructed of a series of cells, each having separate parietes, and 

 extending from the dermal surface to near the inner surface of the 

 Sponge, where they discharge the fcecal streams into the cloacal cavity. 



In Leucosolenia the system of cells is entirely wanting; the 

 Sponge is composed of a single thin stratum of membranous struc- 

 ture and spicula, surrounding a large cylindrical cloacal cavity, from 

 the terminations of which the streams are discharged. 



Leuconia. nivea, a sessile growing Sponge which occurs massive 

 and irregular in form, contains numerous cloacal cavities, each ter- 

 minating in a single large mouth, the interstitial structures between 

 the sides of these cavities and the dermal surface of the Sponge con- 

 sisting of irregularly disposed membranes and spicula, terminating 

 in orifices or oscula in the sides of the fcecal cavity into which the 

 excurrent streams are discharged. 



In Leucogypsia the Sponge is massive witliout cloaca, and with 

 oscula at the external surface, thus simulating the mode of structure 

 of the Siliceous Sponges, whose pores and oscula are mostly on the 

 surface of the Sponge, the pores being the incurrent and the oscula 

 the excurrent terminations of the canals, conveying the streams of 

 water through the Sponge. 



There is a curious phenomena connected with these pores, and 

 that is that they can, when closed entirely, coalesce, so that no trace 

 of them can be found, and fresh orifices are formed at a different 

 part of the surface of the Sponge, afterwards. 



