PLEUROBRANCHUS. 269 
their terminations, which float free for about } of an inch. 
The two portions of the plume fold on each other, with 
fine short cilia on each of the striz, to beat the water and 
eliminate air therefrom; the tout ensemble presents a very 
beautiful branchial plumose appendage, of a pale brown colour, 
aspersed throughout with minute lght flake-brown spots. 
The margins of the ventral disk are often, as in the Bullide, 
reflexed on the dorsal one, and the animal continually twists 
them into various sinuous shapes ; when in full extension they 
float on the surface of the water; it is very probable that the 
large flexible margins of the disk not only minister to a 
creeping locomotion, but also serve for natation. 
In this animal we see an illustration of the doctrine I have 
advanced, that a more elaborate composition of the generative 
organs usually produces greater energy in the motive powers. 
As to the internal anatomy I may observe, that I have dis- 
sected five large individuals, and from the base of the ceso- 
phagus there are four stomachal cavities ; the two first descend 
in a straight line, and form with the other two a right-angled 
turn to the right side, and terminate in an intestine and 
rectum at the posterior right termination under the branchice ; 
the stomachs are pear-shaped, some plain, and others ridged 
at the internal surfaces. The heart lies in a pericardium, 
accompanied by its auricle, near the anterior end of the 
animal, and the right side; it is only a simple ventricle re- 
ceiving the oxygenated blood from the auricle. The liver 
is large, of a green colour, and posterior to it is the light 
yellow granular suboval ovary: the salivary filaments have 
some resemblance to those of Dentalium Tarentinum, but are 
fixed much further from the mouth than in that animal. 
There is nothing particular in the nervous cordon of two 
gangha, from which the threads, in large animals, may be 
observed by the naked eye radiating to all parts. 
These animals are frequently met with in the coralline zone 
in summer, and in the winter are often washed on the Warren 
Sands, at Exmouth, in considerable numbers. 
