856 



SPOXGES AND ZOOPHYTES 



essentially the same; and the peculiarity that chiefly distinguishes 

 the sponge-colony from the plant-like colonies of the flagellate 

 Infusoria is that whilst the latter extend themselves <mttr,i,-<Ix \>\ 

 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 amoeboid cells, through which a current of water is 

 drawn in to meet them by the action of the flagella. The minute 

 pores (fig. 653, b, b) with which the surface a. a of the living sponge 

 is beset lead to incurrent passages that open into chambers lyiiii^ 

 beneath it (c. c), and open into the ' ampullaceous sacs.' or, as thev 

 are now called, 'flagellate chambers,' from the presence round 

 their walls of the flagellate or collared cells. The water drawn 

 in by their agency is driven outwards through a system of 

 excurreiit canals, which, uniting into largerjtrunks, proceed to the 



oscvla or projecting 

 vents, d. from each of 

 which. during the 

 active life of the 

 sponge, a stream of 

 ^ water, carrying out ex - 

 crementitious matter, 

 is continually issuing. 

 The in-current brings 

 into the chambers 



both food-material and 

 *IG. 658. Diagrammatic section of a sponge: , a, n xvo-pn -.11, 1 fvmn +1 1Q 

 superficial layer; I, inhalant apertures or pores ; r, c, OX ^8 en > andtiom the 

 flagellated chambers; fl, exhalant oscule ; e, deeper manner in which 

 substance of the sponge. coloured particles ex- 



perimentally diffust ( I 



through the water wherein a sponge is living are received into its 

 protoplasmic substance, it seems clear that the nutrition of the entire 

 fabric is the resultant of the feeding action of the flagellate units, 

 each of which takes in, after its kind, the food-particles brought by 

 the current of water, and imparts the product of its digestion of them 

 to the general sarcodic mass. 



The continuous substance that clothes the skeleton of the 

 sponge and constitutes the chief part of its living body includes 

 great numbers of stellate granular cells. Their long slender pseudo- 

 podia., radiating towards those of their neighbours, often unite 

 together to form a complex network; on the chief parts of the 

 course of the water-way they become fusiform in shape and con- 

 tractile in function, and it is by their agency that the continual 

 contractions and expansions of the oscula are produced, which are 

 very characteristic of the living sponge. As was tirst shown by 

 Professor C. Stewart, sensory organs, formed of groups of cells 

 >\ith long projecting filaments, are to he seen on the surface of 



lll:lll > s | ges. Any one of these a imrlx .ids. again, detached from 



the mass, may lay the foundation of a ne\\ ' colony.' Jn the 

 aggregate mass produced by its continuous segmentation certain 

 globular clusters are distinguishable, each having a cavity in 



