* 



DEVELOPMENT OP SPONGES. 395 



of Ehrenberg's Allotreta, together with those in the stomach 

 of the Rotifera and Planaria ? 



"The 'swarm-spore,' described by M. N. Lieberkiihi^ 

 appears to be a ciliated form of the seed-like body, and 

 the same as the ' gemmule ' described by Grant ; but this 

 I have not yet been able to see. The formation of the 

 seed-like body, however, now that we know the struc- 

 ture of the ampullaceous sacs, seems very intelligible, 

 for we have only to conceive an enlargement of the small 

 sponge-cells lining the interior, with the addition of 

 ovules to them respectively, and the spicule-bearing 

 sponge-cells of the cortical substance supplying the 

 spicular crust to the exterior, to have a globular capsule 

 thus composed, with a hilum precisely like the seed-like 

 body a conjecture which seems to derive support from 

 the fact, that in some instances, when Spongilla is begin- 

 ning to experience the want of nourishment, these sacs, 

 small as they are, assume a denned, rigid, spherical form, 

 from their pellicle becoming hardened and encrusted with 

 extremely minute spicules." 1 



Clionce. Not the least wonderful circumstance con- 

 nected with the history of sponges, is the power possessed 

 by certain species of boring into substances, the hardness 

 of which might be considered as a sufficient protection 

 against such apparently contemptible foes. Shells (both 

 living and dead), coral, and even solid rocks, are attacked 

 by these humble destroyers, gradually broken up, and, no 

 doubt, finally reduced to such a state as to render sub- 

 stances which would otherwise remain dead and useless in 

 the economy of nature available for the supply of the 

 necessities of other living creatures. 



These boring sponges constitute the genus Cliona of 

 Dr. Grant. They are branched in their form, or consist of 

 lobes united by delicate stems ; they all bury themselves 

 in shells or other calcareous objects, preserving their com- 

 munication with the water by means of perforations in the 

 outer wall of the shell. The mechanism by which a crea- 

 ture of so low a type of organisation contrives to produce 

 such remarkable effects is still doubtful, from the great 

 difficulties which lie in the way of coming to any satis- 



(1) Ann. of Nat. Hist., July, 13. r >7. 



