544 ORGANS OF RESPIRATION. 



surface, the branchial vessels destined for the aeration of the 

 blood. 



The cephalopods are also entirely marine and branchiated 

 mollusca, but with the palleal cavity open anteriorly, and the 

 branchiae, symmetrically developed on the two sides, are 

 suspended by peritoneal and muscular folds along the dorso- 

 lateral part of the abdominal cavity. The respiratory currents 

 enter by the anterior lateral valvular openings of the mantle, 

 and after bathing the branchial laminae, they escape from the 

 palleal cavity by the median valved orifice of the syphon. 

 The large respiratory cavity is separated from that containing 

 the nutritive and generative organs, by muscular aponeuroses 

 and folds of the peritoneum. The branchiae have a pyramidal 

 form, with the free apex directed anteriorly ; they consist of 

 numerous superimposed laminae, diminishing in size ante- 

 riorly ; and they support the trunk of the branchial artery 

 along their fixed upper margin, and the branchial vein along 

 their pendent inferior free border. There are two of these 

 organs on each side of the body in the nautilus, consisting, 

 as on the left side of the pectinibranchiate gasteropods, of a 

 larger and a smaller branchia the bilateral symmetry esta- 

 blished in the cephalopods, and the median position of the 

 syphon, allowing of the equal development of these organs 

 on both sides of the body. In the argonauta, another testa- 

 ceous form, there is but one branchia on each side of the 

 palleal cavity, and the same structure is seen in all the naked 

 cephalopods ; these organs vary, however, in their relative 

 magnitude, and in the number of laminae of which they are 

 composed^ in different species. There are only about twelve 

 laminae in each branchia of octopus* about fifteen in argo- 

 nauta, about forty in sepia, and as many as ninety are some- 

 times found in one of the long narrow branchiae of loligo, 

 the branchial laminae being generally smaller and narrower 

 in proportion to the increase of their number. 



The branchial circulation is accelerated, and consequently 

 the extent of respiration increased, by the force of the two 

 lateral muscular cavities of the heart ; and the arterialized 

 blood is sent by the branchial veins to the median single ven- 

 tricle, for distribution through the body. The branchial lami- 

 nae, supported each around a central cartilaginous hoop like 

 the arches of the gills in fishes, and minutely subdivided into 



