PORIFERA. 



67 



Fig. 71. 





a, c, d, Spicula of Tethea Cranium ; d, three 

 forked spicula; c, fusiform spicula; a, cuticular 

 spicula; b, spicula of Tethea Lynciirium. (After 

 Johnston.} 



cavity within them, extending from the one 

 point to the other ; and on the inflated part 

 of each spiculum we observe a ragged open- 

 ing, as if a portion had been driven out by 

 the expansion of some contained fluid. In 

 those spicula which had suffered little change 

 of form by their incandescence. I have never 

 failed to observe the same cavity within ex- 

 tending from one end to the other, and a dis- 

 tinct open rent on their side by which the 

 contained matter has escaped." The exist- 

 ence of this central cavity has likewise been 

 recognised by Mr. Bowerbank, who, more- 

 over, observes, that it is " lined with an ani- 

 mal membrane, which becomes converted into 

 a thin film of carbon when the spicula are 

 exposed to the action of the blow-pipe." 



Gelatinous cortex." In the recent and living 

 sponge, all its canals and pores are filled with 

 a glairy colourless fluid like the white of an 

 egg, which flows freely out on the removal of 

 the sponge from the water. The quantity of 

 this fluid varies according to the species. In 

 some, it is copious even to nauseousness, but 

 in the compact Halichondrise, there is little of 

 it, and in the Grandse it appears to be en- 

 tirely wanting." * It " has an unctuous feel,, 

 emits a fishy odour when burnt, leaves a 

 thin film of membrane when evaporated, and 

 appears to the naked eye, transparent, colour- 

 less, and homogeneous, like the white of an 

 egg : but when a drop of it is examined on a 

 plate of glass under the microscope, it appears 

 entirely composed of very minute, transparent, 

 spherical or ovate granules like monads with 

 some moisture. These monad-like bodies, 

 nearly all of the same size and form, resemble 

 the pellucid granules or vesicles which Trem- 

 bley has represented as composing the whole 

 texture of the Hydra, or the soft granular 

 matter we observe in the stems of living Ser- 



* Johnston, loc. cit. 



tulariae ; and indeed most of the fleshy parts 

 of organized bodies appear to be composed of 

 similar pellucid granular or monad-like bodies 

 in different states of aggregation."* The sen- 

 sible qualities of this glairy material vary in 

 different species of sponge, " the odour of .some 

 being decidedly animal, while others belong to 

 common and well-known vegetables. The 

 Spongia coalita, when newly taken out of 

 the water, smells very strongly of the com- 

 mon mussel, and when burnt it still resembles 

 the same bivalve burnt ; the Spongia com- 

 pressa, on the other hand, smells strongly 

 of the common mushroom ; some, as the 

 Spongia oculata, have scarcely a perceptible 

 odour." 



Irritability. According to Audouin t and 

 Milne Ed wards, when a living Tethea is allowed 

 to remain for some time perfectly undisturbed 

 in a vessel of sea-water, its oscula may be 

 observed widely expanded, and the currents, 

 hereafter spoken of, passing through them 

 may be readily observed. But if, in this con- 

 dition, the animal is disturbed or removed for 

 an instant from the water, the currents grow 

 much feebler, or cease altogether, and the 

 oscula, contracting slowly and insensibly, be- 

 come at last almost obliterated. In other 

 genera of sponges, however, this contraction 

 has been looked for in vain ; and although 

 the openings of the oscula have been watched 

 with the utmost attention, and measured at 

 intervals with miscroscopic accuracy, not the 

 slightest movement has been perceptible. 



Circulation of Water. In the living sponge, 

 as was first remarked by Professor Thomas 

 Bell, and subsequently by other observers, 

 a constant circulation of the surrounding ele- 

 ment, is, by some mysterious agency, kept up 

 throughout its substance, the water being 

 perpetually sucked in, as it were, through all 

 the minute pores, upon the periphery of the 

 mass, and again emitted in continuous streams 

 through the larger orifices (oscula) of the 

 sponge.. 



Fig. 72. 



Living Papiflaris, shou-ing the jets of water emitted 

 from the oscula. (After Blainville.} 



Dr. Grant put a small branch of Spongia 

 coalita with some sea water into a watch-glass, 

 in order to examine it with the microscope, 

 and thus describes the phenomena it pre- 



* Grant, loc. cit. 



f Hist. Nat. du Litt. ik- la France, vol. i. p. 78. 



F 2 



