OF THE SPONGIADJE. 185 



of a portion of the same section taken at a, fig. 363, X 

 108 linear. 



DICTYOCYLINDRTJS, Sowerdank. 



Skeleton. Without fibre. Composed of a loosely com- 

 pacted columnar axis of spicula, disposed principally 

 in the direction of the line of the axial column, from 

 which a peripheral system of long single or fascicu- 

 lated defensive spicula radiate at right angles to the 

 axial column. 



Type, Dictyocylindrus hispidus, Bowerbank. 



Halichondria Jmpida, Johnston, and Spongia stuposa, 

 var. damicornis, Montagu, are excellent types of the peculiar 

 mode of arrangement of the spicula which characterises 

 this genus. The skeleton consists of a central column of 

 large elongate spicula, disposed principally in the line of 

 the axis of the sponge and at a slight angle to it, approach- 

 ing in form an irregular cylinder of network of elongated 

 meshes, rarely exhibiting an appearance of horny fibre, but 

 formed for the most part of spicula cemented together near 

 their terminations. Towards the base of the sponge the 

 horny substance surrounding the spicula is sometimes so 

 thick as to simulate a proper horny fibre ; but if it be 

 carefully traced, it will always be found to be dependent 

 on the spicula ; where their course is abruptly terminated 

 the horny structure also terminates ; whereas in true horny 

 fibrous structures which contain spicula the course of the 

 fibre is continuous and uniform whether the spicula be 

 present or deficient, and in the newly produced fibre the 

 latter is generally the case. 



The structure of the skeleton in this genus differs from 

 that of Halichondria oculata, Johnston, (Chalina oculata, 

 Bowerbank), in the regularly elongate disposition of the 

 spicula of the skeleton ; and the spicula are necessarily 

 very much larger and longer than those included in the 



