﻿SPONGES.-HALLMANN. 



283 



This species is represented by two specimens, the larger of 

 which is 100 mm. high, 30 mm. in diameter at the base, and 

 65 mm. in diameter near its upper extremity where it is 

 broadest. The lamella? are thin and somewhat parchment-like, 

 and, in keeping with the sym- 



metrical shape of the sponge, 

 display a marked uniformity 

 of arrangement, being so dis- 

 posed that their planes lie 

 parallel to imaginary lines 

 radiating upward with an 

 outward curvature from the 

 sponge-base to the free 

 periphery. The superficial 

 lamellae enclose between them 

 usually hexagonal "cells," 

 averaging 4 mm. in diameter ; 

 occasionally, however, owing 

 to the incompletion (or non- 

 formation) of intervening 

 lamellce, several adjoining 

 "cells" may, so to speak, 

 run into one. In the interior 

 of the sponge the lamellae are 

 relati\ely long and thus give 

 rise to a tubular structure ; 

 in the outer portions of the 

 sponge they are comparatively 

 short and form a more open 

 reticulation. 1 



With regard to skeletal 

 characters, there is nothing of 



importance which might be added to what has been mentioned 

 in the diagnosis, except that the fibres within the lamella? 



Fig. 64 — Echinodathria rotunda. 

 a Principal styles. b Auxiliary 

 strongyles. c Isocliela paluiata. 



1 The regularity of the arrangement of the lamellae in this species 

 enables one to perceive more readily than in the other species herein 

 described, the mode of growth by which the characteristic Ecihnoclathrian 

 structure is attained. The formation of new tissue appears to be con- 

 fined chiefly, if not entirely, to the peripheral region of the sponge, ancT 

 proceeds in two ways— (i.) by the outward growth, at their outer edge, 

 of the superficial lamella?; and (ii.) by the formation of new lamellae. 

 Growth of the first kind increases not only the external dimensions of the 

 sponge, but also, owing to the convexity of its surface, the size of the 

 superficial "cell-apertures." Outward growth of the edge of any given 

 lamella appears to be limited, or, at least, intermittent; and this cessa- 

 tion of growth of now one, now another, of the superficial lamellae, results 

 in the confluence of adjoining "cells," and thus is also, indirectly, a cause 

 of cell-enlargement. The formation of new lamellre counteracts this in- 

 crease in size of the "cell apertures;" each lamella arises, as a tongue- 

 or strap-shaped process, at or near the outer edge of an older superficial 

 lamella— particularly one forming the longer side of an elongated cell— 

 and. growing across the cell aperture, forms at first a narrow septum, 

 dividing it into two. Thereafter, its further increase of size is effected 

 by outward growth along its exterior edge, i.e., in a direction at right 

 angles to its earliest direction of growth. 



