1863.] DR. J. S. BOWERBANK ON THE SPONGILLID^. 453 



opinion as I observed a few of the profusely spiniferous spicula of 

 the envelope intermixed with the proper smooth spicula in the short 

 lengths of skeleton separating the ovaria. The spicula of the enve- 

 lope of the ovary are also cylindrical in form ; hut they are rather 

 shorter and stouter in their proportions than those of the skeleton, 

 and occasionally evince a slight inclination to be subfusiform, and 

 they vary considerably in size. They are slightly curved, and abun- 

 dantly spiniferous, more especially on the outer curved surface and 

 towards the middle of the shaft of the spiculum. They are dispersed 

 very irregularly over the envelope ; and many of them are nearly 

 completely immersed in its strong, coriaceous, cream-coloured sub- 

 stance ; but none of them are visible on its inner surface, which ap- 

 peared quite smooth. The wall of the ovary is much thinner than 

 that of the envelope, and is apparently very little thicker than the 

 length of the short birotulate spicula, which are closely packed in a 

 single layer in all parts of its substance. The spicula of the wall of 

 the ovary are very beautiful objects. The rotulse consist of two 

 thin flat plates of equal size, with entire margins, connected by a 

 very short, thick, smooth, cylindrical shaft, a slight protrusion of 

 each end of which through the centre of each rotula forms a very 

 short convex umbo on the centre of each outer surface. They are 

 very small ; an average-sized one measured, length of spiculum 

 ;, f^\ (J inch, diameter of rotula ywts i^ch, length of the shaft within 

 the rotula y-ffVs inch. 



The gregarious habit of these curious ovaries is very interesting, 

 and the manner in which they are based on the surface of the bark 

 of the plant is very suggestive of the habit. They are not closely 

 adherent by the surface of the envelope, but are supported by a 

 single series of skeleton-spicula, disposed at various angles like a 

 crowd of small props, one end of each spiculum being based on the 

 epidermis of the plant, while the other impinges on the surface of 

 the envelope, the whole being strongly bound together by the horny 

 structure of the skeleton; and short lengths of skeleton-fibre are 

 thrown out laterally, by which the ovaria are connected with each 

 other. It would therefore appear that, as soon as the locality has 

 been selected, the adherent envelope of each ovary generates a single 

 series of spicula to support it above the surface of the plant, so as 

 to allow of a free circulation of water around it during the period of 

 its immersion, and that afterwards they remain dormant in this con- 

 dition until the next season of immersion in the water, as all these 

 little groups were in precisely the same stage of development ; and 

 they were by no means few in number. On one small twig which 

 passed through a specimen of S. reticulata 5 inches in length, and 

 extended 4 inches beyond its termination, there were no less than 

 six of these small colonies, varying from three to twenty-six in 

 number, although the branch did not exceed one-eighth of an inch 

 in diameter at any part. 



Spongilla paulula, Bowerbank. 



Sponge sessile, coating surface rugged, spinous. Oscula simple. 



