856 



SPONGES 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 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 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 lying 

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

 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 

 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 ex- 

 crementitious matter, 

 is continually issuing. 

 The in-current brings 

 into the chambers 

 both food-material and 

 FIG. 653. -Diagrammatic section of a sponge: , a, OX yg e n ; and from the 

 superficial layer ; b, inhalant apertures or pores ; c, <-, J , . , 



flagellated chambers; rl, exhalant oscule ; e, deeper manner ill winch 

 substance of the sponge. coloured particles ex- 



perimentally diffused 



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 first shown by 

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

 with long projecting filaments, are to be seen on the surface of 

 many sponges. Any one of these amceboids, again, detached from 

 the mass, may lay the foundation of a new ' colony.' In the 

 aggregate mass produced by its continuous segmentation certain 

 globular clusters are distinguishable, each having a cavity in 



