RADIATA 



705 



CLASS PORIFERA. 



There can be no question that, if the Sponges and their allies be admitted into the Animal kingdom 

 they must form a distinct group, below the class of Polypes. Not only is the radiated disposition of 

 parts altogether wanting, but even that deSniteness of form is absent, whicli so peculiarly distinguishes 

 the higher groups of Animals from the members of the Yegetable kingdom. The internal structure 

 is no less deficient in Animal characters. There is no stomach or digestive cavity for the reception of 

 the food, — no nervous system or organs of sensation and locomotion, — and notlring beyond the very 

 simplest apparatus for reproduction. No movements of a decidedly animal nature can be observed in 

 them ; the gradual change of form of the orifices of the canals, which is sometimes witnessed, having 

 at least an equal resemblance to the movements of many Plants : neither is there any decided indication 

 of the presence of sensibility. Perhaps the strongest argument in favour of their animal nature is to 

 be found in the resemblance of their structure to the general mass of Alcyonia, which may be likened 

 to a sponge with polype mouths ; and it is an interesting fact that, in the extension of these structures, 

 the spongy mass is the part first produced, the polypes not appearing upon it until a subsequent period. 

 The exterior of every Sponge is covered with minute orifices or pores (whence the name of the 

 class), thickly set together; and between these are seen the larger openings or vents, which, if traced 



downwards into the substance, are found to be 

 the mouths of large canals or vessels that ra- 

 f^ mify through it. The pores open into a less 

 regular arrangement of tubes and minute cavi- 

 ties, of which the spongy mass is principally 

 composed; these freely communicate with one 

 another throughout the mass ; and the canals 

 arise from the midst of them, by small tubes 

 which unite into larger ones, these again meet- 

 ing to form the wide channels which terminate 

 in the vents. Through these canals, in the li- 

 ving Sponge, a constant stream of fluid issues 

 _ „ L « uNuE forth; the supply being kept up by absorption 



tl r ugh the pores The cause of this movement is unUrjwn. Prom these vents also issue forth the 

 reproductive gemmulcs, which are minute ciliated gelatinous bodies, resembhng Animalcules. They 

 are first seen as minute opaque yellow points, irregularly distributed in the gelatinous substance of the 

 body, and usually at some distance from the surface. As their development proceeds, they project 

 from'the walls of the canals into their cavities; and at last become altogether detached, and are 

 carried forth by the current. 



The substance of the Sponge is chiefly composed of tubular fibres of a homy character, which form 

 a network that possesses considerable elasticity ; this network forms, as it were, a skeleton, which is 

 clothed with a gelatinous flesh. In the greater proportion of Sponges, it is strengthened by spicules, or 

 needle-shaped crystals of earthy matter; these, which are sometimes composed ol silex, in other cases 

 of carbonate of lime, are disposed at intervals throughout the mass ; but are especially abundant in 

 the neighbourhood of the canals, and around the external orifices both of these and of the pores, each 

 of which is strengthened by a regular framework of spicules. 



No classification of Sponges that has yet been proposed is likely to have a permanent value ; so 

 little being yet known of their real nature, and of the characters which should serve as the guide in 

 their systematic arrangement. 



CLASS INFUSOPaA. 

 The improvements recently made in the Microscope, and the large amount of attention that has been 

 devoted of late to the natural history of this class, has vastly added to our knowledge of it. One result, 

 however, of this increase of knowledge, which is mainly due to the researches of Professor Ehrenberg, 

 has been to show that the two groups of which the Cuvierian class consists are really very distinct 

 from one another, resembling each other in nothing but their minuteness of size, and the softness and 



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