206 ZOOLOGY 
them from those which are simply diverticula of the body-wall. 
These cerata often coexist with well-developed gills, and their 
minute structure does not point to any very definite respiratory 
function ; it seems not improbable that their varied shape and 
colour may be in some cases protective and in others con- 
spicuous and warning. In those Opisthobranchs which possess 
hepatocerata, such as Doto and £olis, the liver is not a compact 
gland, but consists of a number of diverticula given off from 
the alimentary canal, each diverticulum passing into one of the 
cerata, and being large enough for food particles to pass into 
it and be there digested. In Zolzs the liver diverticula do not 
end blindly, but are stated to open into an invagination of the 
ectoderm termed the cnidophorous sac. This opening is guarded 
by a minute sphincter muscle. The cnidophorous sac, which in 
its turn opens to the exterior, is hned by a number of large cells, 
cnidoblasts, which are crowded with nematocysts or thread-cells ; 
these recall the stinging organs of the Coelenterata. The 
everted threads of these nematocysts are armed with both 
large and small spines. 
In Aplysia, Bulla, and Pleurobranchus the original cten- 
idium has been retained, but is situated behind the heart, in Doris 
and its allies the ctenidium appears in a modified form as a 
circlet of feathered processes which surround the median dorsal 
anus; in Lolis, Tethys, etc., it has completely disappeared. In 
Aplysia there is a large gland, whose secretion is said to be 
poisonous, which opens just below the osphradium near the 
anterior end of the ctenidium; and numerous small cutaneous 
glands open on the under surface of the mantle, in Aplysia 
these produce a purple secretion. 
The Opisthobranchs are, like the Pulmonata, hermaphrodite ; 
and the generative organs consist of a hermaphrodite gland or 
ovo-testis. Some of the cells of this gland form ova, whilst 
others divide up and become spermatozoa. From the ovo- 
testis a hermaphrodite duct leads to an albuminiparous gland, 
in the substance of which the cuct coils. Just where the 
duct leaves the gland it gives off a small diverticulum, the 
vesicula seminalis. The duct then passes on to the external 
opening, but just before it reaches that it receives the duct of 
a spherical spermatheca. When eggs leave the body by means 
