302 
ZOOLOGICAL LITERATURE. 
is described in detail (pi, 4. figs. 17 & 18) j her§ the new 
cell is built upon the top of the old one, Figures and details Rre 
given of the nervous system of EucraUa chelata, Lepralia m- 
tida, Flustra membranacecLj Bcrupocel]ar\a scruposaj and Bugula 
fcistigiata ; the details giyen of the nervous system are among the 
most important parts of this memoir. 
Sexual Reproduction 
Is known to exist in the Polyzoa under very peculiar conditions. 
In Scrupocellaria scruposa (pi. 6. figs. 8-14, pi, 7. figs. 1 & 2), for 
example, during July and August, the first appearance of ova in 
the ovisacs is in the form of two or four clear and pellucid vesi- 
cles, which lie close to each other enclosed in a thin membrane ; 
there is no trace of a nucleus. As the ovum grows, its contents 
acquire a yellowish colour, and an excentric shining nuclear 
body makes its appearance. Other ova make their appearance, 
but there is generally a great difference in the age of the ova 
met with in a single ovisac. After the nuclear body is formed 
the cell-contents become thinly granular; and then there is im- 
mediately noticed a germ vesicle ; it goes on growing and becomes 
invested with an epithelial-like coating of cells, probably secreted 
by the ovisac. At this stage impregnation takes place, the sperma- 
tozoa being formed in the lower part of the cell : they first make 
their appearance as a collection of free round cells ; these increase 
in number though little in size, and at last fill up the whole of 
the lower fourth part of the cell as extremely minute and fine 
threads, without distinct heads, and moving in a wriggling 
manner ; they then swarm to the upper part of the celh where the 
ova lie, It is most difiicult to follow the change in the ovum 
after impregnation ; but segmentation aud the development of 
cilia was distinctly observed. A similar process was observed in 
Flustra memhranokcea (pi. 7. figs. 3 & 4). Sometimes no distinct 
definite testis is met with, and then the loose cell-mass from 
which the spermatozoa are formed, being in the cavity of the 
body, presents a most striking resemblance, both in appearance 
and in the mode of its formation, to the Fettkroppar. 
These Fettkroppar have been known to be characteristic con- 
tents of the chylaqueous fluid of the lower animals, and subser- 
vient to their nutrition. We can now, however, see their connexion 
with reproduction : nor is this only met with in the Polyzoa. 
Budge has shown the development of spermatozoa in Scenuris 
tuhifex from such bodies ; Carter has shown the same in some 
of the Naides ; the same is probably true in many of the An- 
nelids; and Wagner has lately shown how these Fettkroppar 
may, even in insects^ larvse {Cecidomya) j be applied to repro- 
ductive purposes. 
