48 STRUCTURE AND PHYSIOLOGY 



The gemmules are all clothed uiili vibratile Fig. 6. 



cilia similar to those of the tentacula of the 



parents. Sometime previous to their dis- / <^j M \ r^ 



charge from the polypidom 'they are observ- I / I f) \J J 



ed to be in a constant rotatory motion, by 

 which their birth seems to be facilitated ; and now at liberty in 

 the water they move and swim about as if they were guided by 

 volition and sense, whirling at the same time on their own axis.* 

 This freedom to move whither they hst may continue for several 

 hours, or even for two or three days, before a proper site for 

 their permanent stay and future growth is found, when they 

 begin to shoot up rapidly into those beautiful forms particular 

 to each species, as the Supreme Being has ordered and deter- 

 mined. The transformation of the ova, says Di' Grant, " from 

 their moving, irritable, and free condition of animalcules, to that 

 of fixed and almost inert zoophytes, exhibits a new metamor- 

 phosis in the animal kingdom, not less remarkable than that of 

 many reptiles from their first aquatic condition, or that of insects 

 from their larva state." One purpose of this mobility in the 

 ova is obvious ; — it is a means ordained for their diffusion, for 

 the parents being fixed immoveably to one spot, the reproduc- 

 tive germs would have dropt and sprung up at their roots, had 

 thev not, bv some such mechanism as we have described, been 

 carried to a distance, and spread over the bosom of the deep. 



The evolution of the gemmules, subsequent to their fixation, has 

 been minutely traced by Professor Grant and Sir J. G. Dalyell. 

 When the bud falls from the crested head of Tubularia indivi- 

 sa, slight prominences, enlarged at the tips, pullulate from the 

 under surface, and the " nascent animal" elevating itself on 

 these rudiments of the tentacula, as on so many feet, enjoys the 

 faculty of locomotion. " Apparently selecting a site, it reverses 



♦ In reference to those of Flustra carbasea — and tlie observation appears to 

 be very generally applicable — Dr Grant says — " they are very irritable, and are 

 frequently observed to contract the circular margin of their broad extremity, and 

 to stop suddenly in their course when swimming ; they swim with a gentle glid 

 ing motion, often appear stationary, revolving rapidly round their long axis, with 

 their broad end uppermost, and they bound straight forward, or in circles, with- 

 out any other apparent object, than to keep themselves atloat till they tlnd them- 

 selves in a favourable situation for fixing and assuming the perfect state." — Edin. 

 New Phil. Journ. iii. 117. 



3 



