54 FORMS OF THE POLYPIDOMS. 



there is an evident tendency to its formation ; the materials lie 

 scattered in the form of spicula in the soft gelatinous centre; 

 and in the Cydonium these spicula have become so numerous, 

 that they impart considerable rigidity to the whole mass. 



The polypidoms of the Hvdraform and Ascidian zoophytes 

 are more diversified in their fiijures and more decidedlv arbores- 

 cent. The latter are formed by an aggregation of distinct cells, 

 united in general after the fashion of the quincunx and spread 

 out into leaves or layers or compressed branches ; or the cells 

 being placed upon each other in pairs, or even in a single line, 

 they form neat confervoid tufts ; or lying immersed without any 

 very traceable pattern, the masses resulting from their union are 

 amorphous, or at least inconstant and irregular. The horny 

 material of the Hvdra tribe isalwavs formed into tubular sheaths 

 encasing the living flesh, jointed at intervals, sometimes of the 

 same calibre throuohout, but more commonlv dilated at intervals 

 into vases or cups, or cells, in which the proper body of the 

 polype is placed. The manner in which the sheath or tube is 

 divided and branched, is limited in diversity only by the num- 

 ber of the species, which are among the most delicate and in- 

 terestincr of all polypidoms, and pre-eminently imitative of vege- 

 table forms. These forms are of course altogether independent 

 of their aniniated tenants, — these " have been specifically ap- 

 pointed by Him to do what they have done, and are still effec- 

 tuating. They are meie instrumentalities at His conmiand. 

 They know nothing of the results they cause, nor mean to per- 

 form any of them, nor could of themselves co-operate with each 

 other, nor produce any systematical arrangement, or regulated 

 or orderly effects. It is their Master and Maker who organizes, 

 governs, and guides them to those movements and operations 

 which they perform, and from all others ; so that by His direct- 

 ing will they are made to do what we see them effect, and that 

 only because He restrains and averts them from all else."* 



The formation of polypidoms has been the subject of con- 

 siderable discussion. The opinion of Ellis, as we have already 

 seen, was, that they are the result solely of a transudation, or 

 excretion of the constituent matters from the body of the poly- 

 pes, and this opinion has been maintained recently by Lamarck, 



" Turner's Sac Hist, of the World, Vol. ii. p. 71. 



