SPONGES AND ZOOPHYTES. 117 



CHAPTER XIII. 

 SPONGES AND ZOOPHYTES. 



I. SPONGES. 



508. THE determination of the real character of the animals of this 

 Class has been entirely effected by the Microscopic examination of their 

 minute structure; for until this came to be properly understood, not only 

 was the general nature of these organisms entirely misapprehended, but 

 they were regarded by many naturalists as having no certain claim to a 

 place in the Animal Kingdom. It may now be unhesitatingly affirmed 

 that a Sponge is essentially an aggregate of Protozoic units, of which 

 some correspond in every particular to the collared Flagellata (Fig. 295), 

 whilst others resemble Amcebce (Fig. 289), the two conditions being 

 probably only different stages of the same life-history. These units are 

 held together by a continuous sarcode-body, which clothes the skeletal 

 framework that represents our usual idea of a Sponge. In the simpler 

 forms of sponges, however, this framework is altogether absent; in 

 others it is represented only by calcareous or siliceous ' spicules/ which 

 are dispersed through the sarcodic substance (Fig. 352, B); in others, 

 again, the skeleton is a keratose (horny) network, which may be entirely 

 destitute (as in our ordinary Sponge) of any mineral support, but which 

 is often strengthened by calcareous or siliceous spicules (Fig. 352, A); 

 whilst in what may be regarded as the highest types of the group, the 

 siliceous component of the skeleton increases, and the keratose dimin- 

 ishes, until the skeleton consists of a beautiful siliceous network re- 

 sembling spun-glass ( 511). But whatever may be the condition of the 

 skeleton, that of the body that clothes it remains essentially the same; 

 and the peculiarity that chiefly distinguishes the Sponge-colony from the 

 plant-like colonies of the Flagellate Infusoria (Fig. 296), is that whilst 

 the latter extend themselves outwards by repeated ramification, sending 

 their zooid-bearing branches to meet the water they inhabit, the surface 

 of the former extends itself inwards, forming a system of passages and 

 cavities lined by these and the amreboid zooids, through which a current 

 of water is drawn-in to meet them by the action of the flagella. The 

 minute pores (Fig. 351, #, b) with which the surface a, a, of the living 

 Sponge is beset, lead to incurrent passages that open into chambers lying 

 beneath it (c, c}; and it is especially on the walls of these ' ampullaceous 

 sacs/ that the flagellate zooids present themselves. The water drawn-in 

 by their agency is driven outwards through a system of excurrent canals, 

 which, uniting into larger trunks, proceed to the oscula or projecting 

 vents d, from each of which, during the active life of the Sponge, a 

 stream of water, carrying out excrementitious matter, is continually 



