B8 ANATOMY AND PHYSIOLOGY 



as ill the genus Dactylocalyx. These are but a few of the 

 numerous varieties of form that exist in the order Silicea, 

 but as the whole of them will be described in detail in the 

 course of the characterisation of the genera, it is unneces- 

 sary to enter further into a description of them at present. 



The membranous structures as applied to the composition 

 of the skeleton assume generally a much less prominent 

 position than the previously described part, yet in some 

 few genera they are really the principal element. Thus in 

 Hymeniacidon they are the primary part of the structure, 

 and the spicula dispersed over their surfaces are the sub- 

 sidiary portions only ; and in Microciona Hymerapkia and 

 Hymedesmia the basal membrane is the indispensably 

 necessary part of the structure. 



SARCODE 



Is a pellucid, semi-transparent gelatinoid substance, 

 variable in colour and insoluble in water. It dries readily, 

 and its physical characters are restored by immersion in 

 water with little or no apparent alteration. It is usually 

 spread thinly and rather evenly over the internal tissues, 

 but the surface is rarely perfectly smooth ; sometimes it 

 abounds in obtuse elevations, and occasionally separates 

 naturally into innumerable irregularly round or oval masses 

 which are exceedingly variable in size. When examined 

 by transmitted light with a microscopic power of 400 or 

 500 linear, it is always found to contain innumerable 

 minute molecules of apparently extraneous animal or 

 vegetable matter, the molecules being always more or less 

 in a shrivelled or collapsed condition, and very variable in 

 size. Occasionally it is found abundantly furnished with 

 lenticular nucleated cells, nearly uniform in size, and often 

 highly coloured. Fig. '285, Plate XVI, represents a portion 

 of the interstitial membrane of the honeycomb sponge of 

 commerce, with the sarcode in its natural condition, filled 

 with the remains of the nutrient molecules in a collapsed 

 state. Fio-s. 281 and 282, Plate XVI, exhibit the same 



