ORGANS OF RESPIRATION. 543 



dorsal ramified branchial tufts, a distinct pulmonary sac, 

 lined with vascular network, is observed, opening by a single 

 spiracle, above the anus, on the median line, at the posterior 

 end of the body ; and both kinds of organs exhibit rhithmic 

 respiratory movements of dilatation and contraction, in their 

 respective elements. Some pulmonated gasteropods, breath- 

 ing solely by lungs, live in the fresh waters, as lymnaa and 

 planorbiSy the exterior surface of whose body, and the mucous 

 lining of the alimentary canal, are covered with vibratile cilia, 

 as in the branchiated marine species ; others are terrestrial, 

 and testaceous, as helix, or naked, as Umax, and these likewise 

 breathe solely by means of a large single pulmonary cavity 

 placed in the middle of the back, and opening by a single 

 small round spiracle on the right side, near to the head. The 

 interior surface of this sac is highly vascular, from the numerous 

 large reticulate branches of the pulmonary arteries and veins 

 there distributed ; the aerated blood is transmitted to the 

 auricle and ventricle, to be sent through the system, as in 

 other mollusca ; and the external aperture of the respiratory 

 organ is here approximated to the anus, as in onchidium, and 

 as these two passages are in most of the internal forms of 

 respiratory organs of invertebrata. And thus, in the air- 

 breathing gasteropods, the highest pulmonated invertebrata, 

 the lungs already form a circumscribed cavity, extended 

 along the middle of the back, with one tracheal opening, as 

 the air-sac of fishes and the embryo lungs of all higher 

 vertebrata. 



All the pteropods are marine, branchiated, swimming 

 mollusca, with a closed cavity of the mantle necessitating an 

 external disposition of the branchiae, and whose naked sur- 

 face, or extensive palleal fold enveloping a thin shell, may 

 assist in the aeration of the body. The branchiae of the 

 small naked pneumodermon form two considerable crescentic 

 laminated organs extending freely from the surface of the 

 mantle at the caudal extremity of the body. In the testa- 

 ceous hyalea, they form an elliptical ring of branchial laminae 

 disposed on the back, between two folds of the mantle, and 

 the lateral apertures of the shell appear to transmit the 

 respiratory currents to this open cavity of the mantle. In 

 clio and cimbulia, the natatory fins, developed from the sides 

 of the neck, appear to support, on their striated or laminated 



