60 
POPULAR SCIENCE REVIEW. 
Such is the class in general character. I proceed to notice 
the points of special interest to which I referred at first. 
The increase of the polyzoan community by external budding 
is one of the most familiar facts in its history ; but it is only 
lately that we have learnt that a provision exists for the renewal 
of the polypides, and that as one generation of them disappears 
its place is soon filled by another, which occupies the same 
cells and perpetuates the life of the colony. In the Hydroid 
Zoophyte the life of the individual polypites is evanescent ; 
they soon perish, and are followed by others which bud from 
the common flesh. Amongst the Polyzoa also it appears that 
a succession of zooids may occupy the same colony, but the 
mode in which the second generation is supplied differs widely 
from that which obtains amongst the Hydrozoa, Students of the 
former class have long been familiar with certain dark-reddish 
bodies, of somewhat spherical form, which are commonly present 
in the cells, and remain in them after the death and disappear- 
ance of the polypides. Not unfrequently whole colonies of 
BoiuerbanJda for instance are found, in which almost every 
cell is tenantless, but exhibits a single dark spot. 
These bodies have been commonly regarded as ova, which are 
not liberated till after the death of the polypide ; but they have, 
as we shall see, a totally different significance, the discovery of 
which has opened to us one of the most deeply interesting 
chapters in the polyzoan life-history.* Let us first trace them 
to their origin, and then enquire into their function. At certain 
seasons a very marked change is seen to be taking place towards 
the base of the body of the polypide (fig. 1, i). This change 
consists in the gradual separation of the lowest portion of the 
stomach from the rest of that organ. After a time a somewhat 
spherical dark-reddish mass is found to be, as it were, suspended 
below the stomach, with which it is still connected by a narrow 
channel. In this semi-detached portion the characteristic con- 
traction of the walls, by which the food is driven upwards within 
the digestive cavity, may yet be noticed. At this stage there is 
no absolute separation, but merely a contraction of the stomach, 
by which its lowest portion is formed into a distinct chamber. 
At length, but whether before or after the death of the polypide 
I am unable to say, this inferior section of the digestive sac is 
completely cut off from the rest and lies within the cell as a 
separate structure. In this condition it is found to consist of a 
granular mass enveloped in a membrane, which is thickly covered 
* The interesting facts which I am about to detail were first fully 
brought to light by the Swedish naturalist Smitt, in a very able paper “on 
the Development of the Marine Bryozoa.” Ilis observations have been con- 
firmed by Nitsche; and I have myself verified them again and again. 
