226 
POPUIAR SCIENCE EEVIEW. 
described ; from these they spread forth their wreaths of 
tentacles in search of food, and here they find shelter when 
alarmed. They exhibit a very simple structure ; the body is soft 
and contractile, containing an ample digestive cavity, hollowed 
out, as it were, in its substance, which communicates through 
an orifice heloiv with the general cavity of the coenosarc, and 
above terminates in an oral aperture, around which are ranged 
in a single series a number of filiform tentacles. The polypite 
is in fact a laboratory of the simplest construction, in which 
nutriment is prepared for distribution through the common- 
wealth. The only approach to complexity of organisation is 
found in the prehensile arms, which are not mere extensile 
threads for entangling prey, but are furnished with an armature 
of poisoned darts, and not only arrest but paralyse their 
victims. 
If we examine the soft body-substance of the zoophyte, 
we find it to consist of two layers, an outer {ectoderm) and an 
inner (endoderm), the latter lining the whole of the interior 
cavity, and the former constituting the outer integument. These 
elements enter into every part of the structure. The common 
flesh that pervades the horny corallipe, and upon which it has 
been moulded, is a simple tube bounded by these two layers ; 
the body of the polypite is a somewhat enlarged extension of 
this tube, slightly modified, and the tentacles are slender pro- 
longations of it. The endoderm is chiefly concerned with 
nutrition, and its inner surface bears the multitude of cilia, 
which by their incessant movements maintain the circulation of 
the fluids in the general cavity of the body. The ectoderm is a 
structureless layer of contractile substance, closely resembling 
the sarcode of inferior organisms, and betraying its affinity with 
it in certain genera, by sending out extensile processes, like 
any Rhizopod, which can be completely withdrawn into the 
masses from which they originate. Such is the building mate- 
rial of which the Hydroid organism is composed. In the outer 
layer are lodged the organs of offence, which are known as 
thread-cells, and which are very characteristic of Coelenterate 
structure. Mounted on prominent nodules distributed along 
the tentacles, and concentrated in batteries at various points of 
vantage, these formidable projectiles give the Hydroid colony 
immense facilities for the capture of food. The thread-cells 
consist of minute sacs embedded in the flesh, and enclosing long 
{Thecaphoi'a). In anotlier division {Athccata) the polypitea are naked, though 
the common flesh is more or less invested by a horny covering or polyparj' ; 
while in the remaining sub-order {Chjmnocliroa), of which the Hydra is the 
sole representative, there is no hard integument whatever, and the poly- 
pites are single and locomotive. 
