THE TUNICATA. , 32] 



strument, it alone feels the injury, and retires, without any 

 others being conscious of the danger, or of the hurt inflicted on 

 their mate. The polyzoa propagate by gemmation and by ova 

 or eggs, which, germinating on the inner surface, escape at a later 

 period into the visceral cavity, and are finally discharged into 

 the wide sea, so to fulfil their mission in creation, and people the 

 shores of every clime with myriads of busy workers in horn and 

 in lime, which, with subtle chemistry, they draw from a fluid 

 quarry and build up in textures of admirable beauty and 

 heaven-ordered designs." 



Each polyzoon begins with a single ovum. The original or 

 seminal cell of a fiustra or lepralia has no sooner fixed itself 

 upon some stone, shell, or alga, than new buds begin to shoot 

 forth, which in their turn produce others from their unattached 

 margins, so as rapidly to augment the number of cells to a very 

 large amount. Thus a common specimen of Fiustra carbasea 

 presents more than 18,000 individual polyzoa, and as each of 

 these has about twenty-two tentacula, which are again furnished 

 with about a hundred cilia3 a piece, the entire polyzoary pre- 

 sents no less than 396,000 tentacula and 39,600,000 cilise. The 

 Rev. David Landsborough calculated that a specimen of Fiustra 

 membranacea five feet in length by eight inches in breadth had 

 been the work and the habitation of above two millions of in- 

 mates, so that this single colony on a submarine island was about 

 equal in number to the population of Scotland. As the tentacula 

 are numerous in this species, four thousand millions of cilise 

 must have provided for its wants, about four times the number 

 of the inhabitants of this globe ! 



The Tunicata are so called because their soft parts are not 

 enclosed in a calcined shell such as invests the majority of their 

 class, but in a more or less coriaceous envelope or tunic which 

 is either bag-shaped, and provided with two apertures, or tube- 

 shaped, and open at the ends. They present a strong resem- 

 blance to the Polyzoa, not merely in their general plan of 

 conformation, but also in their tendency to produce composite 

 structures by gemmation ; they may, however, be at once dis- 

 tinguished from them by the absence of the ciliated tentacula 

 which form so conspicuous a feature in the external aspect of a 

 fiustra or a retepore. Their branchiae, which have generally 

 the form of ridges (e), occupy a large sac, forming, as it were r 



