GILLS 



327 



the great length of the single filament is considered, and the fact that there 

 is an exposure of the blood to the oxygen supply along its entire length. 

 A characteristic form of respiratory membrane is to be seen in the gill 

 of the lobster. A long axis with nervous, muscular, and other structures 

 bears a stream of blood out to its end and sends it off into a series of 

 filaments. In cross section, such a filament is oval (Fig. 292), and its 



ep. lam. 



bl. ca. 



&-- ?bl. ca. 



FIG. 292. Part of a transverse section of a respiratory filament of the lobster's gill, bl.v., 

 afferent and efferent blood vessels; bl.ca., capillaries; hy., hypodermal (epidermal) cells 

 which line the body surface and produce the cuticle; cu., cuticle; ep.la.rn., thin layer of epi- 

 dermal cytoplasm lying between a respiratory capillary and the cuticle; conn.t., connective- 

 tissue nuclei ; x, unknown bodies near wall of one blood vessel. Arrows show how blood 

 passes from artery (afferent blood vessel) to vein (efferent vessel). X 725. 



outer edge is formed by a layer of covering cells in a loose mass of con- 

 nective tissue containing the blood vessels of the filament. There is 

 no basement membrane to separate the hypodermis from the connective 

 tissue. 



All blood spaces in the filaments appear to be channels lying among 

 the Leidig's connective-tissue cells, and they are of two groups. First, 

 there are the afferent artery and an efferent vein that together form a 



