234 B. W. PRIE8T ON THE NATURAL HISTORY 



of the lien's egg. It is tliis last process that may be considered as 

 internal gemmation, and, according to Dr. Grant, if two or more 

 ova are placed on a watch glass or slide, they spread themselves out 

 as a thin transparent convex circular film, which, if they come in 

 contact with one another, coalesce, thicken, and produce spicula, 

 which in a few days grow up as from one ovum. 



In Tethya leyncvrium true gemmules, according to that term, are 

 produced externally. At the base of most specimens a small glo- 

 bular mass of sarcode may often be noticed, in which, if mounted in a 

 cell in Canada balsam, the skeleton spicules, like those of the parent 

 sponge will be plainly made out. 



There is an interesting paper, with plate, in the December number 

 of the " Journal of the R.M.S. for 1880," treating on this subject of 

 the external gemmation of Tethya, dredged up in the White Sea by 

 a German naturalist. It appears that this budding process does not 

 confine itself to the base only, but occurs at any part of the Sponge, 

 giving it the appearance of being covered with some parisitical 

 growth. 



The third process of reproduction is by division of the sarcode, 

 viz., if a portion of the sarcode, naturally or even by artificial means, 

 be separated from the parent sponge, it will ultimately develope into 

 l^erfect form. 



This process that Sponges have of growing from detached por- 

 tions has been recently taken advantage of by two German 

 naturalists, on account of the supposed likely falling off of the supply 

 of the Sponges of commerce. They found that by carefully cutting 

 small pieces from living specimens, and attaching them to boards or 

 stones, and sinking them in suitable localities and depths, they 

 will grow into large Sponges as if nothing had happened. 



The genus Spongilla has been long known to develope by both 

 ovaria and division of sarcode, but generally in the latter case, very 

 few, if any, ovaria could be found present. 



If two pieces of living Spongilla be placed under suitable circum- 

 stances so as to touch, they will, after a certain time, coalesce in the 

 same manner as described with regard to the ova. 



Tliore is still much to be found out regarding the growth and re- 

 j)roduction of Sponges, as also the process which enables certain 

 sjjccies to burrow into shells and substances of a hard and compact 

 nature. One often picks up oyster shells completely riddled by one 

 species named Cliona (now changed to Jlymcniacidon) cciata, and up 



