POLYPI. ]'y3 



polypes themselves are not developed till after the 

 formation of the root and stem ; their growth being 

 in this respect analogous to that of the leaves and 

 flowers of a plant : but this analogy must be imder- 

 stood with reference only to the mode of develope- 

 ment, for it in no respect extends to the functions 

 of those vegetable structures. 



The gemmules of the Flustra carbasea may be 

 selected in illustration of these phenomena. These 

 have been observed by Dr. Grant,* to swim about 

 in the water as soon as they have escaped from the 

 cells of the parent ; each moving w ith its narrow 

 end foremost, while the opposite broad end, whicli 

 is covered with cilia, expands into a flat circular 

 zone. These gemmules are very irritable, and are 

 frequently seen to contract the circular margin of 

 their broad extremity, and, while swimming, to stop 

 suddenly in their course. They swim with a gentle 

 gliding motion ; at other times they appear sta- 

 tionary, all the while revolving rapidly round their 

 longer axis, with their broad end uppermost : they 

 often bound forwards, either in straight lines, or 

 describing circles, with no other apparent object 

 than to keep themselves afloat, until they shall 

 arrive at a favourable spot for flxing their perma- 

 nent abode, and proceeding in their further deve- 

 lopement. The time of their remaining in this 

 free and moving state varies according to circum- 



participate, during the life of the animal which has formed them, in 

 the vitality of the soft parts, and that they are even capable of 

 growth by a kind of intus-suscejition ; that is, by the introduction of 

 fresh materials through the pores of the substance itself. Ann. Sc. 

 Nat. serie 2. vj, 25, and x, 321, 334. 



* Edinburgh Philosophical Journal, XVII. 107 and 337. 



