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34 



THE SPONGES. 



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The specimens are of interest in that the cavity of the sponge is 

 crossed by one or more horizontal or oblique septa. The septa (Fig. 4, 

 Plate 4) are obviously similar structures to the well-known " bottom 

 plate/' which has been observed in several species of Euplectella (Schmidt, 

 1880, p. 60; Schulze, 1895, p. 17; Schulze, 1902, p. 53; Ijima, 1901, 

 pp. 40, 92, 103, 207). The septa are spongy, easily broken, and com- 

 posed of a reticular tissue formed by the continued deposition of silica 

 chiefly round small diacts, with the development of synapticula. The 

 diacts discernible in the beams of the reticulum have rounded and slightly 

 enlarged ends. The proper skeletal strands of the sponge wall take no 

 share in forming the septa. 



In the specimen figured the septa are arranged one above the other. 

 In the other specimens there is but one septum. The septa are thicker 

 at the periphery, thinning out toward the centre. Except in one case 

 they are perforated by several rounded apertures, about the size of the 

 parietal apertures. In the case referred to, the septum is imperforate, but 

 is exceedingly thin in spots. On its upper surface the reticular beams 

 have a predominantly radial disposition, and moreover are vaguely divided 

 into groups, each of which radiates from a particular part of the body 

 wall. This arrangement suggests that the septum arises as a number of 

 centripetal outgrowths from the wall, the outgrowths meeting and coalesc- 

 ing. And in fact, in the specimen figured, three independent outgrowths 

 of this character, all lying in the same transverse plane, exist above the 

 uppermost septum. 



The reticular tissue of the septa, at the periphery of the latter, is 

 prolonged as a thin unevenly developed layer over the adjacent parts of 

 the sponge wall. In some of the specimens, in which there are no septa, 

 this reticular layer is developed over extensive areas of the gastral sur- 

 face, and in a less degree over the outer surface. In such places the 

 tissue is thick enough to hide completely from view the proper composi- 

 tion of the wall, passing over and so closing the parietal apertures. 



The union of the smaller parenchymalia into a finely reticulate tissue, 

 which in the one case spreads over the surface of the sponge, and in the 

 other crosses the paragastric cavity, would seem to be everywhere 

 essentially the same phenomenon, perhaps having a definite physiological 

 function. The structure of the "bottom plate " in the specimens studied 

 is similar to that of the septa. 













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