BRITISH SPONGIADiE. 23 



together, forming a common point, and within this hollow 

 cone of spicula the distal termination of the interstitial cell 

 may be observed considerably elongated, gradually at- 

 tenuated at that part of its parietes where the proximal 

 ends of the ciliary spicula are attached, and this gradual 

 inclination of the side of the cell towards its long axis 

 necessarily effects a corresponding inclination of the circle 

 of ciUary spicula, thus producing by the simple collapse of 

 the distal end of the cell the same effect that would have 

 been achieved by muscular action in animals of a higher 

 order of structure. In the contrary condition of the ciliary 

 circle of defensive spicula, where they are all projected in 

 lines parallel to that of the central axis of the cell, and forming 

 an open cylindrical tube, we find the part of the cell to 

 which their proximal ends are attached in a fully expanded 

 condition, and the extreme distal end of the cell terminating 

 hemispherically, and sometimes more obtusely. 



Thus, by the simple and natural act of the inhalation of 

 water and consequent distension of the distal extremity of 

 the interstitial cell, the cone of spicula is expanded into a 

 cylinder, and the ready access of nutritive particles to the 

 pores is promoted ; but the moment that action becomes 

 languid, or ceases altogether, a collapse of the cell ensues, 

 and the distal points of the spicula again approach each 

 other. The distal ends of the interstitial cells are not in 

 contact with each other until slightly below the points of 

 attachment of the proximal ends of their defensive ring of 

 spicula, and in their semi-collapsed condition they bend over 

 in any direction with perfect facility, and this amount of 

 flexibility is thus an effectual safeguard to these delicate but 

 yet brittle defensive organs, the spicula of which are stouter 

 and very much shorter than those surrounding the mouth 

 of the cloaca. 



The equiangular and triradiate spicula of the skeleton 

 vary considerably in size and form, and one or two of the 

 three rays are frequently more or less curved, to adapt 

 them to the purpose of their position, and occasionally one 

 ray will be considerably longer than either of the other tw6. 

 I have described and figured the anatomical pecuUarities of 



