248 BULLETIN OF THE BUREAU OP FISHERIES. 



COURSE OF THE BIvOOD IN THE GILL. 



The description of the course of blood through the gill given above usually 

 suffices for the text-books of zoology, but the physiologist wishes to know how the 

 blood circulates in the gill filaments, for if these were simple capillary tubes it would 

 tend to flow past rather than through them. The gill in reality is a complicated struc- 

 ture, and the actual course of the blood is not easy to follow." 



Each filament, like the stem of the branchia, is a double tube or vascular loop, 

 consisting of outer afferent and inner efferent divisions (fig. 2, pi. xi,vii.) All the blood 

 must pass from the afferent branchial vein (o/. v.) to the afferent divisions of the loops, 

 thence to the efferent divisions, and then to the main efferent of the stem (e/. ■z;.). The 

 wall of the branchial afferent vein which carries unaerated blood to the filament sug- 

 gests a cylindrical sieve or grater, with fine holes arranged in regular transverse rows. 

 As the blood enters one of these holes it is conducted by a short passage to the afferent 

 division of the loop or filament, but, as Dahlgren and Kepner have shown, the course by 

 which the efferent half of the filament is reached is indirect. The venous blood in the 

 afferent section enters a plexus of fine channels or capillaries, by which it is conducted 

 around the filament and into the efferent loop. In the course of this passage the venous 

 blood is brought close to. the cuticular surface, but never quite touches it, there being 

 always a cytoplasmic layer of the true epidermis of the filament, from which the cuticular 

 covering is supplied at each successive molt. Thus, in passing through the filament the 

 blood is kept in close relation to its surface, a condition which tends to promote the most 

 active exchange of gases essential to respiration. These capillaries do not, apparently, 

 have definite walls, but worm their way between or through the cells. The connective- 

 tissue cells of the central core of the filament are described by Dahlgren and Kepner 

 as being essentially peculiar and characteristic in possessing loosely branched proto- 

 plasmic processes. The efferent channel of each filament empties into a circular vessel 

 (fig. I, pi. XLVii, c. V.) which runs around the main afferent of the stem, and thus 

 conveys the arteriaUzed blood to the efferent vein {ef. v.). 



The course of the blood through the gill is thus, in brief, as follows : Stem afferent 

 to filament afferent, through filament capillaries to filament efferent, to circular vessel in 

 wall of stem afferent, to stem efferent, to branchio-cardiac vein, to pericardium and heart. 



This system of vessels is filled with blood, which, owing to the rhythmic contractions 

 of the heart and the dispositions of its valves, is kept moving in the same direction, 

 from heart to tissues, from tissues to gUls, and from giUs to heart again. The heart is 

 "arterial," and the breathing organs of the crustacean are thus introduced into the 

 returning stream of venous blood, the converse of the conditions found in fishes, where 

 the heart is "venous" and the giUs participate in the arterial system which leaves it. 



o The account of the drculation of blood in the gill g^iven in this section was written six years ago, when the drawings illus- 

 trating it were made. Certain details concerning the capillary plexus have been added since reading the work of Dahlgren and 

 Kepner, who, so far as we are aware, were the first to describe the histology of the filament and the course of the blood throughit 



