412 STRUCTURE AND ECONOMY OF TETHEA. 



velopment of these bodies, which I would thus state pro- 

 visionally : — A spiculu/m is formed by a series of sponge-par- 

 ticles, so arranged in a column or system as, by the siliciflcation 

 of more or less of tJieir contiguous surfaces and substance, to form 

 a continuous rod. 



Mode of union of different parts of Skeleton. — Horny matter 

 of the same nature as the peculiar substance which forms the 

 entire skeleton of the keratose sponges, but more delicate and 

 transparent, binds together the spicula, and connects the 

 fasciculi of the central portion of the framework. 



The diminished quantity and greater delicacy of the 

 horny connecting matter in the Tethea now under consider- 

 ation, as well as in Tethea cranium, appears to me to depend 

 on the greater contractility of the soft texture of these sponges. 



Towards the periphery of the intermediate or radiating 

 masses, the long spicula are bound together by numerous 

 fibre-cells, which may be contractile, but from their thin and 

 dry aspect, I am inclined to consider them as ligamentous, 

 and consequently better adapted to the movements of the 

 various parts of this sponge than the elastic but more rigid 

 horny texture. 



The minute curved spicula, which are not very numerous, 

 are always to be found arranged across the long spicula ; and 

 as their length nearly corresponds to the breadth of a pair of 

 the latter, and as I have not unfrequently seen them hooked 

 on in this position, I am inclined to consider them as connec- 

 tive structures. 



Contractility and contractile textures of Tethea. — I have 

 not had a sufficiently favourable opportunity of observing the 

 contractility of Tethea. I have, however, no doubt whatever, 

 from the recorded observations of Donati and of Audouin, and 

 Milne Edwards, that these sponges are capable of slow but 

 decided contractions and relaxations. In the outer part of 

 llni recently-formed rind the fleshy substance consists of a 



