TETHIUM. — Body subglobular, irregular, tuberiform, sarcoid but firm, suberous, re sisting, supported by and mixed up with an immense quantity of aciculi, which are simple, fasciculate and diverging from the centre to the circun:ference.
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 sponge. These spicula, which are composed of pure ailex, 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 the nature and arrangement of the solid por tions that the general characters of the mass depend. In the true sponges (Spongia), 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 vvith 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 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. Bower bank* and others, the horny fibres being, in fact, solid and imperforate.
In a second group of Sponges, called Halichondria (xa24 silex ; xoyapos, carti lage), the solid framework of the body is principally made up of silicious spicula, bedded in the fibre or parenchyma of the species present us with spicule of two differ ent forms, though they sometinaes 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.t A third group of Sponges, designated by Blainville, Caleespongia, has the framework which gives them form solidified by the pre sence of spicula, which are entirely coniposed 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. (fig. 71, d). The calcareous spicula are more variously shaped —either simple and acicular 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.$ " When
these spicula are examined through the micro scope after exposure to a red heat, we dis tinctly perceive," says Dr. Grant §, " a shut 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 likevvise 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 Halichondrim, there is little of it, and in the Grantim 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 tularim; 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 f and Milne Edwards, when a living Tethea is allowed to remain for some thne 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 altogethtr, 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.