328 HISTOLOGY 



loop to carry the blood into and out of the filament as was done in the 

 dogfish embryo. Our figure shows one half of a section through a fila- 

 ment, and one of the large blood vessels, the artery. 



But this vascular loop does not form a direct pathway. The second 

 set of vessels are a set of fine capillaries that serve to carry the blood from 

 the artery to the vein along their entire course. They leave the artery 

 on its outer edge and extend around, as a plexus lying close to the surface, 

 to empty into the outer side of the vein. They thus keep a large amount 

 of the blood close to the surface, and in a suitable position for gas ex- 

 change or respiration to go on. 



These capillaries appear to have no walls of their own, but to pass 

 between the connective-tissue cells, between these and the hypodermis 

 cells, or even between the hypodermis cells and the cuticle. They never 

 quite reach this cuticle, however, as a small plate of cytoplasm belonging 

 to the hypodermis cells always keeps them from directly touching it. 

 This is well shown at ep. lam. in Figure 292. 



The connective-tissue cells that form the central core of this filament 

 are more characteristic than any others in the lobster's body. They do 

 not show the periphery that the Leidig type of cell does, but have loosely 

 branched protoplasmic processes. A very peculiar set of round objects 

 which somewhat resemble nuclei are found on the inner side of the artery 

 and are shown at x in Figure 292. 



All the blood vessels, even the small capillaries, are lined with a very 

 thin cuticular substance which gives them a clear and unmistakable 

 outline that is well shown in the drawing. It should be remembered 

 again that the surface of this gill consists of the same elements that the 

 crustacean or insect body does, of an epithelium or hypodermis which 

 secretes a cuticle, here modified by thinning for a special purpose. The 

 organ is evaginated because it it to be used in water. If it were to be 

 used in air, it would be invaginated as it is in the insects. 



Note the thin layer of cytoplasm lying between the blood capillary and 

 the cuticle at ep. lam. in Figure 292. This is not to keep the blood from 

 touching the cuticle, but to provide the cuticle with a portion of cytoplasm 

 which is the only agent which can make it and renew it when necessary. 



Some of the gills found on worms are remarkable structures, and 

 bear interesting histological relationships. The gill filaments of the 

 worm, Amphitrite ornata, are good examples and a transverse section 

 of one of these long, extensible filaments will show the desired features 

 (Fig. 293). These filaments are used for other purposes than respira- 

 tion, and it is not known whether the animal would perish at once, for 

 lack of oxygen, without them. It probably would not, but would have 

 .time to regenerate them. 



As can be seen, the filaments consist of a long core of connective 



