66 



PORIFERA. 



fasciculate, and diverging from the centre to 

 the circumference. 



Skeleton. The framework, or fibrous 



portion, from the arrangement of which the 

 sponge derives its form is composed, as we 

 may gather from the preceding table, of va- 

 rious materials differently disposed in different 

 species, and it is upon the modifications in 



Fig. 68. 



A. minute portion of the surface of Tetliea Cranium 

 magnified; spicula projecting beyond the surface. 

 (After Johnston.) 



the nature and arrangement of the solid por- 

 tions that the general characters of the mass 

 depend. In the true sponges (Spongiei), so 

 remarkable for their elasticity and softness, 

 and for their capability of absorbing fluids, 

 properties which render them valuable for 

 many important uses ; the whole substance is 

 composed of horny subcylindrical fibres, which 

 ramify and interlace in every possible direc- 

 tion, anastomosing with each other so as to 

 form innumerable continuous cells and intri- 

 cate canals, the walls of which, in the recent 

 sponge, are crusted over with the gelatinous 



Fig. 69. 



sponge. These spicula, which are composed 

 of pure sile.r, are generally united into fasciculi 

 by an enveloping glutinous or condensed cel- 

 lular substance, and by the junction of these 

 fasciculi in various modes fibres are formed, 

 which traverse every part of the body, form- 

 ing the boundaries of canals and orifices, and 

 giving form and support to the whole of the 

 gelatinous or soft cellular substance of the 

 animal.* The spicula, so far as the British 

 species are concerned, Dr. Johnston observes, 

 seem to be always in the shape of simple 

 needle-like crystals (fig. 70) ; nor does any 



Fig. 70. 



Single interspace or open cell, and surrounding finer 

 mesh-work of the skeleton of Euplectella Aspergillum. 

 (After Owen.) 



living cortex. The horny threads composing 

 this inextricable labyrinth are of unequal 

 thickness, and by some writers have been 

 erroneously described as being throughout 

 tubular ; but this latter is a mistaken view of 

 their structure, dependent upon optical ap- 

 pearances, as has been proved by Mr. Bovver- 

 bank * and others, the horny fibres being, in 

 fact, solid and imperforate. 



In a second group of Sponges, called 

 Halichondria (xaAi?, silex ; x" 5 /'y> carti- 

 lage), the solid framework of the bodv is 

 principally made up of silk-ions spicula, im- 

 bedded in the fibre or parenchj ma of the 



* Microscopic Journal, vol. i. p. 10. 



A minute film of the rind of Tethea Lyncurium com- 

 pressed between plates of glass, and highly magnified 

 to show the needle-like and starred spicula. (After 

 Johnston.) 



species present us with spicula of two differ- 

 ent forms, though thev sometimes vary much 

 in length and gracility ; but he cannot assent 

 to the opinion of Dr. Grant that the form is 

 different in every distinct species, otherwise 

 the task of distinguishing them would be com- 

 paratively easy.-f- 



A third group of Sponges, designated by 

 Blainville, Calcespongia, has the framework 

 which gives them form solidified by the pre- 

 sence of spicula, which are entirely composed 

 of carbonate of lime : in sponges belonging to 

 this group there is, according to Dr. Johnston, 

 no net-work, their basis being a porous mem- 

 brane, rendered compact by the profusion of 

 spicula imbedded in it. The siliceous spicula 

 belonging to the preceding group form mostly 

 needle- like spines ; but there are found along 

 with them, in the genus Tethea, some that 

 might have been the model from which mytho- 

 logical painters might have drawn the trident 

 they have placed in the hands of Neptune. 

 (fg. 71, rf). The calcareous spicula are more 

 variously shaped either simple and acicnlar 

 or clavate, or formed with three, or even some- 

 times with four prongs. The two kinds, viz. 

 the calcareous and siliceous, have not hitherto 

 been detected co-existent in any British 

 sponge ; but the spicula of every species are 

 very constant to the same figure, although in 

 point of size they vary exceedingly. J " When 

 these spicula are examined through the micro- 

 scope after exposure to a red heat, we dis- 

 tinctly perceive," says Dr, Grant , " a shut 



* Grant, Conip. Anat. p. 5. 



f British Sponges, p. 89. 



j Johnston, loc. cit. 



Edin. Phil. Journ., xiv. p. 184. 



