PLEUROBRANCHUS. 269 



their terminations, which float free for about J of an inch. 

 The two portions of the plume fold on each other, with 

 fine short cilia on each of the striae, 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 light flake-brown spots. 

 The margins of the ventral disk are often, as in the Bullida, 

 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 oeso- 

 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 branchiae ; 

 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 

 ganglia, 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. 



