4 Pol. 
POLYZOA. 
sheath, the stomodaeum, proctodaeum, superior cells of the tentacles, and 
the ganglia. The endoderm produces the mesenteron, and with it the 
brown mass is associated ; this is probably a stomachic caecum, modified 
and containing a quantity of detritus. The mesodermic cells may be 
described as funicular. The muscles are divided histologically into three 
groups, which are described. The reproductive organs are mesodermip ; 
their products are discharged into the cavity of the zooecium, and thence 
into the tentacular sheath. Three types of larvae are distinguished and 
described. (1) The calcareous skeleton of the Bryozoa is deposited 
between the cells of the ectoderm, which exists throughout the life of the 
zooecium, and presents the appearance sometimes of a single layer under 
the skeleton ( Mem branipora ) , sometimes of a double layer enclosing the 
skeleton ( Leprcilia ). (2) The cavity of the body presents mesenchy- 
matous products, according to Hertwig’s classification, and has never any 
endothelial covering. (3) In the Cheilostomata the sucker forms the 
basal wall of the zooecium, and produces the stolon in Vesicularia. The 
new members of the colony are only formed by budding upon these 
derivatives of the sucker (except in the case of the opercular avicularia of 
Cellularia and Eschar ellci). (4) The polypide is formed at the expense 
of the ectodermal rudiment and of the brown body. At the com- 
mencement of its embryonic poriod the larva still presents a special 
organ, designated as “ calotte,” and destined to form this rudiment ; 
OSTKOOUMOFF (4). 
The ovary of Bugula calathus is attached to the neural wall of the 
sexual individual ; a struggle for existence takes place among the ova, 
which are thus reduced to one, or (rarely) to two. The ovary then becomes 
free, a follicular remnant forming the germ of a new ovary. The egg, 
now independent, passes neuralwards to the ovicell ; fertilization is pro- 
bably internal. The brood capsule arises as two diverticula from the 
distal wall ; these grow together, and enclose a space within which 
development occurs ; a layer of peculiar cells is found where the embryo 
is attached ; muscles are also present. Segmentation takes place first in 
the short axis, the next plane is also meridional, the third equatorial ; 
then two planes appear parallel to the first, and two others to the second. 
At this stage four cells may be seen within the blastocoel — the rudi- 
mentary endoderm — which are probably introduced by cpibole. They 
increase in number and form also the mesoderm ; cavities are developed 
in this mass of cells, and give rise to the coelom. An equatorial annular 
thickening takes place, giving rise to the ciliated corona ; and two invagi- 
nations take place on the oral surface, one of which is the suctorial pit, 
for the attachment of the larva. The second invagination is not the 
mouth, though often called so ; it may be termed “ anterior ectodermic 
groove.” At the aboral pole a thick flattened disc intrudes for some dis- 
tance into the anterior of the embryo ; a circular invagination surrounds 
this, and a pear-shaped organ results, regarded as a gland. When ready 
for metamorphosis, the larvae swim round and round, and from place 
to place, during which motions the discoid organ is protruded. The 
attachment, however, does not take place by means of this, but by the 
oral pole ; the aboral side of the larva enlarges, and almost all the dermal 
