58 
POPULAR SCIENCE REVIEW. 
the Tunicata and Brachiopoda , to the rank of a sub-kingdom. 
This, as its author admits, is a merely provisional step ; and it 
is quite possible that, as the result of yet further investigation, 
they may merge once more in the primary division from which 
they have been separated. It is but lately that the general 
plan of the nervous system has been determined, and we are 
still far from a thorough comprehension of its details.* 
It is not my purpose, however, to discuss the systematic 
position of the group, but simply to notice a few of the more 
interesting points in its history, and especially to place before 
the readers of the Popular Science Peview some of the most 
important results of recent research. 
And, as a preliminary, it may be useful to give a slight 
sketch of the form of structure which is characteristic of the 
Polyzoa. In the first place, they are universally composite 
animals. The primary zooid, the imme- 
diate product of the egg, gives origin at 
once by budding to a second, which remains 
in organic union with it ; and this multipli- 
cation by continuous gemmation proceeds on 
a definite plan, but to an indefinite degree, 
during the lifetime of the animal. In this 
way colonies of greater or less extent are 
formed, in which a multitude of zooids ( poly - 
pides) combine individual independence with 
subservience to a common life. 
Each polypide is enclosed in a cell (fig. 
1, a), which is horny or membranous or 
calcareous, as the case may be, and is fur- 
nished with an aperture, usually placed at 
one extremity, through which the polypide 
protrudes at pleasure the anterior portion 
of its body, bearing a wreath of ciliated 
tentacles (fig. 1, 6), in the centre of which 
is placed the mouth. This aperture is, in 
some cases, closed by a movable lip, and in 
others by a much more elaborate operculum. 
The incessant and vigorous play of the cilia creates a perfect 
whirlpool within the tentacular crown, by which alimentary 
particles are borne through the central mouth into a wide 
• It is worthy of remark, in this connection, that the important discovery 
of a il colonial nervous-system,” uniting and bringing into relation the many 
Eooids composing a polyzoan community, which we owe to Fritz Muller, and 
of which we shall have more to say hereafter, is barely referred to by 
Huxley in his recent work, and is dismissed with the remark that it requires 
continuation. 
