AND HISTOLOGY OF SPONGES. 233 



be said to be as yet in its infancy, although things look very much 

 in its favour. 



Much has been written on the reproduction of the Sponges, but 

 in a paper like this it would take too long to go into the subject 

 minutely. 



They are, up to the present time, known to propagate by three 

 modes — 



1st. By ova with and without ovaria. 



2nd. By internal and external gemmation ; and 



Srd. By spontaneous division of the sarcode. 



Dr. Bowerbank tells us that little is known regarding the repro- 

 duction by ova without ovaria, as that models confined to the genus 

 Spongia (the Sponges of commerce) alone, and we seldom obtain 

 them in a fit condition for the examination of that subject ; they may 

 be seen occasionally in specimens that have not been cleaned by the 

 dealers, dispersed evenly over the surface of the fibres, and also im- 

 bedded in the sarcode that coats them, but even then in a very un- 

 satisfactory manner. The ova generated in ovaria are, perhaps, best 

 studied in the freshwater species of the Upper Thames. I name 

 that place more particularly, on account ot the luxuriance of the 

 growth of both the species, Spongilla fluviatilis and S. lacustris, 

 which may be met with growing together at Henley, Goring, 

 Marlow, &c. 



Spongilla fluviatilis grows in a more massive and compact form 

 than S. /acustris, but both are of a bright green colour, and anyone 

 not knowing what they were would certainly take them at first sight 

 as a vegetable growth. The ovaria are found at the base of the Sponge, 

 having the appearance of small brown seeds. When examined under 

 the microscope, after soaking them in spirits of turpentine for some 

 hours, the walls of these ovaria are found to be strengthened and 

 supported by bi-rotulate spicula in lines from the centre to the 

 circumference. Fig. 2, Plate XVII. These ovaria contain the ova, 

 which, as the spring comes on, creep out of the vents, which may 

 be seen situated at one or other part of the ovaria, swimming about 

 by means of cilia, until they at length settle down in some suitable 

 place, and develope into Spongilla. 



In some species of Sponges, as in Grantia compressa for instance, 

 the ova are often developed in the interior of the Sponge, and in that 

 case may frequently be watched, passing through the process of 

 cleavage, similar to the segmentation which takes place in the yolk 



