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 figures and more decidedly 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 Hydra tribe is always formed into tubular sheaths 
encasing the living flesh, jointed at intervals, sometimes of the 
same calibre throughout, but more commonly 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- 
teresting of all polypidoms, and pre-eminently imitative of vege- 
table forms. These forms are of course altogether independent 
of their animated tenants, — these “ have been specifically ap- 
pointed by Him to do what they have done, and are still effec- 
tuating. They are mete instrumentalities at His command. 
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. 
