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POPULAR SCIENCE REVIEW. 
series of facts in the history of the Hydroid. The difference 
lies simply in the mode of gemmation. The hydroid Medusa 
buds from the side of the polypite, or of a special zooid, or of 
some portion of the common flesh ; the Medusa of the Disco- 
phore is a bud formed by the transverse division of the body 
of the polypite. And it may be remarked that this mode of 
budding is not altogether unknown amongst the Hydroids, for 
the Hydra has been observed to multiply by transverse fission. 
The marvel of the staid polypite resolving itself into a com- 
pany of mercurial jelly-fishes disappears ; like the plant, it 
has only put forth its flower-buds, with the difference that they 
open into full bloom and mature their seed apart from the 
parent organism. 
I shall now sum up briefly the chief points of difference 
between the two Orders. Amongst the Discophores, the free 
locomotive element predominates ; the fixed plant-like element 
amongst the Hydroids. The polypite of the former makes 
a nearer approach to the medusan form than that of the latter. 
The sexual zooids (Medusae) of the Discophores possess solid and 
massive discs, with lobed margins and pedunculated eyes often 
protected by hood-like coverings, and a complex anastomosing 
system of vessels ; the opening is rarely provided with a veil, 
and, when present, it is very slightly developed ; the tentacular 
appendages of the mouth attain an extraordinary size, and the 
reproductive organs are lodged in distinct chambers, commu- 
nicating by a definite orifice with the cavity of the body, 
through which the embryos make their escape. On the other 
hand, the free sexual members of the Hydroid colony are 
comparatively small, and have fragile and filmy bells, pro- 
vided with an ample veil ; the vessels are generally simple ; the 
eyes sessile, unprotected, and of a humbler type ; the fur- 
belowed oral appendages are wanting ; and the generative 
products are lodged between the outer and inner wall of the 
digestive sac or of the radiating vessels, and are liberated by 
the rupture of the parts. And, lastly, the mode in which the 
medusiform bodies originate and develop themselves differs 
in the two divisions. Many of these distinctions have little 
special significance ; and though there may be enough amongst 
them to justify us in separating the two groups in our classifi- 
cation, it must be remembered that the affinities are of the 
closest and most intimate kind. 
To resume our history. The polypite having dismissed its 
brood lives on. Thanks to its voracious appetite, it soon 
repairs the waste of its substance, and in the following spring 
may, so to speak, bud and blossom again. The young Medusae, 
which are liberated in a very immature conditijn, pass rapidly 
through the further stages of their development ; so rapidly 
