BRITISH SPONGIAD.E. 313 



the spiculum, there is a slight flexure downward and again 

 upward, so that the apices in the adult spiculum regain the 

 position they would have occupied had their flexures never 

 occurred. But this flexuous bihamate form has a further 

 development ; {imbrications appear on the sides of the 

 shaft, especially towards the hamate extremities, and 

 which are produced to such an extent that they ultimately 

 form two deep terminal pouches, with the mouths of each 

 facing the other, the shaft and the apices of each hook 

 being the two points of support, and the lateral portions of 

 the mouth are distended so as to become nearly or quite 

 circular. Occasionally, the siliceous membranes are decur- 

 rent, meeting at the middle of the shaft. These fully-de- 

 veloped forms were by no means common in the sponge 

 examined, and by far the greater number of the spicula 

 were in a more or less incomplete state of development. 

 The siliceous film of which the pouches are formed is ex- 

 tremely thin and transparent, and requires a careful 

 management of the light to define it well with a power of 

 600 or 700 linear. The spicula are very minute. 



26. Isodictya pygmea, Bowerbank. 



Halichondria palmata, Johnston. 



Sponge. Arbuscular, pedicelled ; palmate, or branching 

 laterally, more or less compressed. Surface even. 

 Oscula simple, dispersed, terminal or lateral. Pores 

 inconspicuous. Dermal membrane thin, pellucid, 

 aspiculous. Skeleton. Slender; primary lines rarely 

 more than bispiculous ; rete, one spiculum wide ; 

 spicula acerate, short, and rather stout. Interstitial 

 membranes. Spicula acerate, slender, few in number. 



Colour. Dried, dull light brown. 

 Habitat. Scarborough, Mr. Bean. 

 Examined. In the dried state. 



