62 
POPULAR SCIENCE REVIEW. 
difficult of detection; but it has already been recognised in 
several of them, and is no doubt present in all. Without at- 
tempting to describe its plan in detail, I may mention that I 
have traced the main nerve-trunk in Bicellaria ciliata , and 
have observed that a branch passes from it to the base of the 
avicularium, which is furnished, as we know, with a special 
apparatus of muscles. The interest and importance of these 
discoveries will be at once recognised.* 
We turn now to the reproductive history of the Polyzoa, 
which offers a few points of special interest. It has long been 
well known that at certain times both eggs and spermatozoa are 
developed in each cell, the former from its membranous lining 
and usually the upper portion of it, the latter towards the 
bottom of the cell. At a certain stage of their growth the ova 
escape from the delicate envelope that has hitherto confined 
them, and lie free in the perivisceral cavity, where they are 
fertilised and enter upon the course of development that 
results in their conversion into active ciliated embryos. Smitt, 
however, has noticed another mode of reproduction, which he 
regards as asexual. He describes eggs as occurring within the 
cell (of Lepralia Peachii ), and lying in a loose mass of fatty glo- 
bules, by which they are nourished. In proportion as the ovum 
increases in size this mass diminishes around it. until at last it 
lies free in the cavity. At this stage it is furnished with a mem- 
brane and is of a dark red colour. It afterwards undergoes seg- 
mentation, and passes into the condition of a ciliated embryo ; but 
no spermatozoa have ever been detected in the cells in which 
such ova are developed. The observation is a verj r interesting 
one, and to a certain extent I can confirm it. But it is to 
be remarked that the spermatozoa are difficult to detect, and we 
require the evidence of repeated observation to assure us of 
their non-existence. Smitt has recorded the occurrence of this 
asexual mode of propagation in several species, and names it “ the 
formation of ova by internal budding.” 
The structure of the curious embryo, which is the immediate 
product of the egg, has been thoroughly investigated by Nitsche 
and illustrated by admirable figures.f It exhibits some very 
interesting points. The shape of the embryo varies in different 
species, and is often difficult to describe, but the plan of struc- 
ture seems to be pretty uniform throughout the class. The 
whole surface of the body is covered thickly with cilia, which 
exhibit some diversities of size and form, and by means of 
which it swims rapidly through the water. Its j movements are 
* Iteichort has challenged Muller’s conclusions ; but his criticism seems 
to be based on the examination of a single species, and is not sustained by 
the results of further investigation. 
f licit rage zur Kcnntniss dcr Bryozoen. Von Dr. Heinrich Nitsche. 1860. 
