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POPULAR SCIENCE REYIEW. 
cular fringes. There they complete their development, and 
finally leave the parental shelter, as ciliated, free-swimming 
planulae. (Plate LXX. fig. 1, the ovum ; fig. 2, the embryo .) 
In this condition they bear an exact resemblance to the cor- 
responding term of the Hydroid life-series. They appear as 
cylindrical bodies, thickly clothed with vibratile cilia, and 
furnished with a mouth at one extremity. After a term of free 
existence the planule selects a site for permanent settlement,'* 1 
and, having made a suitable choice, fixes itself by the pole of 
the body opposite to that which bears the mouth, and ex- 
changes its roving habit for a purely vegetative life (Plate 
LXX. fig. 3). Its shape and proportions have altered ; it is now 
narrowed below, and expands above ; the ciliary appendages 
are still retained for a time, but they have lost their activity* 
while around the extreme basal portion of the body, a very 
delicate film of chitine is in some cases soon developed (Plate 
LXX. fig. 3a). The presence of this horny sheath, which, ac- 
cording to Agassiz, is only characteristic of some species, is a 
very interesting point, as another feature common to the 
Discophores and the plant-like Hydroids. As development 
proceeds the body lengthens, the cilia totally disappear, the 
upper portion becomes broad and somewhat cup- shaped, and 
from its margin bud a number of thread-like arms, while in 
the midst of them rises a quadrangular proboscis, bearing the 
mouth on its summit. The product of the Medusan egg now 
appears as a well-developed polypite, the equivalent of the 
primary zooid of the Hydroid colony (Plate LXX. fig. 4). Yet 
with all the similarity between them, there is an element of 
unlikeness too. The rather deep, cup-shaped disc surrounding 
the prominent proboscis of the Discophore suggests the swim- 
ming-bell of the Medusa, and we feel that if it were turned 
adrift, to float on the water, with the mouth downwards, it 
would bear no slight family resemblance to its parent.f In 
a word, the polypite of the Discophora has more of a Medusan 
look than that of the Hydroids. 
To pursue the history, the polypite multiplies the number 
of its arms until they become a goodly company, corresponding, 
with the voracious appetite which it is their office to satisfy ; 
they are very extensile, and when much elongated are like 
threads of gossamer waving through the water. The mouth is 
* This process of selection, often conducted with much apparent fasti- 
diousness, may he witnessed in the case of the embryos of fixed animals 
generally. 
t It must be remembered that some of the hydroid polypites have the 
tentacles connected by a web-like membrane, plainly indicating the way 
in which the structural elements are modified, so as to convert the fixed 
hydra into the free medusan or sexual zooid. 
