ZOOPHYTA HYDROIDA. 
85 
to believe the polypidom to be extravascular and inorganic— -the 
result of an exudation from the interior pulp, and dependent on 
this for its form and growth, however varied this may be. In the 
reproductive gemmule there are two substances, viz. the pulp 
and the thin cuticle or membrane, the latter of which is the germ 
of the future arborescent or cellular polypidom : by the growth 
of the pulp the membrane is distended and moulded into a cell, 
or pushed upwards in the form of a shoot, in which, after a time, 
the pulp is arrested in its growth longitudinally, swells out, 
and is developed into an animated polype, furnished with ten- 
tacula, a mouth, and digestive organs. Bursting the cell 
at the point which becomes the future aperture, it there dis- 
plays its organs, and begins the capture of its prey, for, unlike 
higher organisms, the polype is at this the period of its birth as 
large and as perfect as it ever is at any subsequent period, the 
walls of the cell having become indurated and unyielding, and 
setting a limit to any further increase in bulk. The growth 
being thus hindered in that direction, the pulp, incessantly in- 
creased by new supplies of nutriment from the polype, is con- 
strained and forced into its original direction, so that the extre- 
mities of the tube, which have remained soft and pliant, are 
pushed onwards, the downward shoot becoming a root-like fibre, 
and the upper continuing the polypidom, and swelling out as 
before, at stated intervals, into cells for the new developement 
of other polypes. — — The polypidom then, however like unto cer- 
tain vegetables in appearance, has, as Ellis said, nothing vege- 
table about it, but is entirely an animal production, and excret- 
ed by the animated pulp which fills its whole interior. Ellis 
and his followers are only so far wrong, that they have attributed 
to the polypes themselves what is actually due to the pulp. The 
latter is that from which the polypidom is exuded ; for, indeed, 
the cell always precedes the existence of the polypus which is 
developed within it ; while the polypidom begins to be develop- 
ed from the gelatinous substance of the reproductive gemmules 
before any polypes are formed, and it continues to be develop- 
ed and extended by the fleshy mass of the zoophyte, whether 
polypes are developed in the cells or not. C4 There is but one 
life, and one plan of developement, in the whole mass ; and this 
depends not on the polypi, which are but secondary and often 
