— 27 — 



inwards by tbe outer wall of tbe dorsal blood-cliamber (Fig. 13, d.a.c). Tbe ventral aü'-cbamber 

 bas correspondiiig relations (v.a.c). In other words, in tbe special part of tbe gill tbere is a Space 

 between tbe general wall of tbe gill (composed of tbe cbitine and hypoderm) and tbe boundary wall 

 of tbe blood-caTity. and tbis Space contains aii-. Tbis Space does not communicate by any opening 

 mtb tbe outside of tbe gill. 



I find tbat in many preparations of sections tbe byiioderm does not appear to be present 

 as a layer lining tbe cbitine in tbe special part of tbe gill (excepting at tbe free end of tbe gill). 

 I am inclined to tbe opinion tbat tbe bypoderm niay become witbdrawn from tbe cbitine and cen- 

 tered in tbe pillars. As already observed, in tbis region of tbe gill tbe bypodermic nuclei are found 

 only as associated witb tbe pillars. It would seem possible tbat sucb a witbdrawal of tbe bypo- 

 derm may take place periodically witb moulting as soon as a new cbitinous layer bas been formed. 



Tbe bypodermic pillars in tbe special part of tbe gill are arranged in rows wbicb divido 

 tbe cbambers into a series of compartments separated by broken walls. Tbe appearance of tbese 

 compartments, as seen wben tbe gill is examined from 'natbout bas already been referred to, p. 47, 

 as being a series of radiating blood cbannels communicating witb tbe main excurrent cbannel. It 

 will l^e understood, bowever, tbat tbe compartments of tbe dorsal blood-cbamber only open directly 

 into tbe main exciu'rent cbannel. 



Pbysiology of tbe outer gill. Tbe course of tbe circulation of tbe blood. 

 Tbe course of tbe circulation of tbe blood in tbe outer gills of Oniscus can be seen by removing 

 tbe first two pairs of tboracic legs, placing tbe animal in water in a watcb-glass, putting on a cover- 

 glass, and examining under a low power of tbe microscope. Fig. 14 is based upon examinations 

 tbus made. Tbe dotted arrows represent tbe coiu'ses taken by tbe in-flowing or venous blood and 

 tbe lined arrows tbe courses taken by tbe out-flowiug or arterial blood. Tbe blood, entering tbe 

 gill at tbe basal Joint, for tbe most part takes tbe patb of tbe main inciu'rent cbannel. But as 

 tbe inner boundary of tbis cbannel is a broken wall (formed by a row of bypodermic pillars) tbe 

 blood constantly escapes from tbe cbannel and spreads over tbe entire region of tbe general part 

 of tbe gill. Tbis distribntion of tbe blood furtber depends upon tbe curvature of tbe outer wall 

 of tbe main in-current cbannel (tbe marginal wall of tbe gill) from wbicb tbe current of blood is 

 constantly being reflected. 



Tbat portion of tbe blood wbicb tbus makes its way to tbe distal region of tbe gill tben 

 turns in its coui-se and enters tbe main excurrent cbannel. The rest of tbe blood passes from tbe 

 general to tbe special part of tbe gill, entering tbe radiating cbannels and proceeding directly out- 

 wards toward tbe margin of tbe latter portion. Here it tui-ns in its course and, foUowing tbe ra- 

 diate cbannels but in tbe reverse direction, enters tbe main excurrent cbannel. A small portion 

 of tbe blood, bowever, does not enter tbis cbannel but joins tbe outflowing stream directly at tbe 

 basal Joint. 



A portion of tbe blood, entering at tbe basal Joint, does not go to tbe general part of tbe 

 gill but to tbe special part and appears to circulate as follows: It tends to spread over tbe region 

 of tbe special part but mingling witb tbe currents flowing from tbe general to tbe special part, 

 its course is gradually lost in tbese currents. 



If now we apply tbe facts of stnictui-e and relation gained by tbe study of tbe anatomy 

 of tbe gill to tbe explanation of tbe course of tbe circulation as observed from witbout, and above 

 described, we are led to tbe following results: 



