. The microscope and its revelations. 4, 13); in other*,again, the skeleton is a keratose (horny) network, which may lieentirely destitute (as in our ordinary sponge) of any mineral support,but which is often strengthened by calcareous or silicious spicules(fig. 654) ; whilst in what may be regarded as the highest type* ofthe group, the silicious component of the skeleton increases, and thekeratose diminishes until the skeleton consists of a beautiful siliciousnetwork resembling spun glass. But whatever may be the condi-tion of the skeleton, that of the body that clothes it remains 1 For an in
. The microscope and its revelations. 4, 13); in other*,again, the skeleton is a keratose (horny) network, which may lieentirely destitute (as in our ordinary sponge) of any mineral support,but which is often strengthened by calcareous or silicious spicules(fig. 654) ; whilst in what may be regarded as the highest type* ofthe group, the silicious component of the skeleton increases, and thekeratose diminishes until the skeleton consists of a beautiful siliciousnetwork resembling spun glass. But whatever may be the condi-tion of the skeleton, that of the body that clothes it remains 1 For an instructive discussion on this point, consult Prof. E. A. JMincliiiis essajdj i The Position of Sponges in the Animal Kingdom in Science Papers, i. ()(1897), to which is appended a useful list of works on the subject. Some authorsdemur to the association of sponges with other Metazoa, and Professor Sollas lui~ suggested the use of the group-name Parazoa. See also Trcul/^r on ^ vol. ,1900. 856 SPOXGES AND ZOOPHYTES. essentially the same; and the peculiarity that chiefly distinguishesthe sponge-colony from the plant-like colonies of the flagellateInfusoria is that whilst the latter extend themselves <mttr,i,-\repeated ramification, sending their zooid-bearing branches tomeet the water they inhabit, the surface of the former extendsitself inwards, forming a system of passages and cavities lined bythese and the amoeboid cells, through which a current of water isdrawn in to meet them by the action of the flagella. The minutepores (fig. 653, b, b) with which the surface a. a of the living spongeis beset lead to incurrent passages that open into chambers lyiiii^beneath it (c. c), and open into the ampullaceous sacs. or, as thevare now called, flagellate chambers, from the presence roundtheir walls of the flagellate or collared cells. The water drawnin by their agency is driven outwards through a system ofexcurreiit canals, which, uniting into largerjtrunks, proc
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