328 ME. A. HANCOCK ON THE ANATOMY 



ones connecting the former, and divided by narrow elongated 

 stigmata. This is tlie true aerating surface of the gill ; and were 

 there no additional appendages, the organ would appear to be 

 composed of numerous transverse series of short longitudinal ves- 

 sels and narrow openings, divided by large transverse channels or 

 vessels ; it would appear to be, in fact, wbat it essentially is. But 

 on first inspection, with the aid of a low magnifying-power, it 

 seems to be formed of a comparatively coarse reticulation of longi- 

 tudinal and transverse vessels of nearly equal size, crossing each 

 other at right angles, and having four or five narrow longitudinal 

 openings or stigmata in each square mesh, dividing as many minute 

 vessels. 



This appearance is produced by the existence of a number of 

 stout so-called longitudinal vessels or bars that extend from one 

 end to the other of the branchial sac, and project considerably 

 from the inner surface of the organ, to which they are attached 

 only at the points where they cross the transverse channels. Here 

 they are supported upon short wide pedicles, through which they 

 receive their supply of blood from these channels ; they are thus 

 lifted some little distance above the general surface of the 

 gill. At these points the longitudinal bars are a little enlarged , 

 and have on their upper margin a stout elongated papilla with the 

 extremity rounded. There is thus a papilla at the angles of each 

 mesh ; and they are all inclined towards the ventral side of the 

 respiratory sac, and have on the upper surface, and in front, an 

 elongated disk, which is apparently ciliated. 



The walls of the longitudinal bars are comparatively thick ; and 

 hence these organs have a certain degree of rigidity. It is not 

 very easy to determine of what use they are ; but perhaps their 

 chief function is to protect the more delicate tissue of the true 

 aerating vascular surface ; while the papillae will conduce to the 

 same end, and by the aid of their cilia probably sweep the sedimen- 

 tary matters towards the oral lamina, the water being beat through 

 the stigmata by the cilia that fringe their borders. From the 

 stiffness of the bars themselves it may be inferred that they will 

 also give support to, and keep stretched out, the vascular network 

 of the sac. They seem ill calculated, on account of the thickness 

 of their walls, to give much assistance in aerating the blood, and 

 are certainly unnecessary as part of the circulatory mechanism. 



The blood, as we have already seen, is brought to and taken 

 from the aerating reticulation by the dorsal and ventral branchial 

 channels, and by nnmei'ons suspenders conneclhig it with the 



