36 
POPULAR SCIENCE REVIEW. 
then all the individuals of the colony assist in the operation. 
There is consequently a will that governs them, a will which 
can only draw the motives of its determination from the exist- 
ence of a sort of social consciousness, raising the whole colony 
to the rank of a psychological unit. Composed of individuals 
each equivalent to one of those Hydrae or Medusae which we 
have seen living freely and sufficient for themselves, every 
Siphonophore must nevertheless be regarded in its turn as a 
single animal, as a true individual of a higher order. Here the 
transformation of the colony into an individual is to a certain 
extent manifest. The Siphonophore is an animal, all the organs 
of which are so many distinct animals, each playing a certain 
part. Moreover, we see these animal-organs gradually become 
less and less independent. They approach others, arrange 
themselves regularly around a central individual, which pre- 
dominates, and finally combine to form a creature, such as the 
For pita or the Velella , in which no one would dream of seeing 
anything but an indivisible animal if the investigation of the 
allied types did not reveal its true nature. 
It is thus, also, that every one hitherto has regarded the 
Sea-anemones and the Polypes of the madrepores and coral, as 
simple organisms, primary individuals, although, in our opinion, 
they owe their origin to a phenomenon exactly like that which 
has produced the Porpitce and the Velellce , and likewise result 
from the union of three kinds of Hydriform Polypes. The fine 
investigations of Moseley on the Polypes of the family Stylas- 
teridm furnish an unexpected proof of this. Considering only 
their calcareous parts, all these creatures seem to be true Madre- 
pores, and the first doubt as to their actual nature was raised by 
Louis Agassiz with reference to the Millepores. 
Between a Coralliarian and a Hydroid Polype the difference 
is considerable ; one is a simple sac, bearing tentacles which 
vary in number with the species, sometimes with the individuals, 
but constant for each of them during the greater part of their 
existence ; these are usually solid, simple appendages of the 
wall of the body. The other consists of a stomachal sac, open 
at the bottom, and around it are arranged hollow tentacles, the 
number of which increases with the age of the polype. These 
tentacles, which have their upper parts free and are united by 
their bases, thus forming the body-wall of the polype, open 
inwards, like the stomachal sac, into a great cavity, the periphery 
of which is divided into as many chambers as there are tentacles 
by the united walls of each two neighbouring tentacles. Upon 
the partitions of these chambers the reproductive apparatus is 
developed, and it thus seemjs to be contained within the body 
cavity of the polype, whilst in general it appears in the 
Hydroid Polypes in the form of an external bud. 
