178 ME. E. J TILLYAED ON THE EEOTAL BEEATHING-APPAEATUS 



however, that this tissue takes a light but veiy characteristic eosin staining, 

 which distinguishes it very clearly from the basal pad. 



The structure of this tissue is the same in all forms examined, though the 

 size and shape of the masses varies greatly. In all cases^ definite cell- 

 boundaries are not easily made out. The masses seem rather to consist of 

 rounded lobules, sometimes with only one nucleus, but more often with 

 several in each lobule. Usually the curved borders of the lobules show free 

 and uncompressed, and there is very seldom seen iniy polygonization of the 

 masses by mutual compression. The nuclei {;nui) are numerous, of moderate 

 size, rounded, darkly staining with hfematoxyliuj and with a fairly large and 

 very distinct nucleolus. 



Owing to its lobulate formation, the hypobranchial tissue never completely 

 fills the space at the base of the gill-fold. It is not, however, therefore 

 necessary to assign anv particular function to the small space left at the top, 

 just below the union of the two gill- walls. Sadones has termed this cavity 

 a blood-space, and based upon it a theory that the blood plays some part in 

 the physiological processes of respiration. Against this, I am able to state 

 definitely that blood-corpuscles never occur in these cavities, and it seems 

 verv doubtful whether they have any regular communication with the 

 hsemocoele at all. There is certainly no regular circulation of fluid in them, 

 nor have I been able to trace in them either any regularity of form or any 

 definite outlets to the hsemocoele. 



The most definite fact about the function of this tissue is that it carries 

 within its meshes, in all cases, the large trachea or tracheoe which enter the 

 bases of the gill-folds. It is thus a support surrounding these main tracheae, 

 and I am inclined to regard this as its principal function. If we remember 

 that the gases received from the water in the rectum must already have 

 traversed the capillary loops and smaller tracheal branches before they reach 

 these larger tracheae, and that, by this time, these gases are simply in process 

 of being transmitted to the main tracheal trunks for distribution throughout 

 the body, we must see that the position of this tissue almost forbids us from 

 ascribing to it any really important part in the physiological processes of 

 respiration. 



As to the origin of the hypobranchial tissue, I should like to state my 

 belief that it will be found to be a special development from the supporting 

 tissue to be found in large masses under the raised epithelium of the so- 

 called '• rectal glands " in the posterior portion of the rectum. The purview 

 of this paper did not, of course, embrace a study of this portion of the 

 rectum. But, in tracing back the posterior ends of the duplex gill-sysfcem 

 in ^-Eschia hrevistyla (text-fig. 8), I had to. follow the gill-basket in section 

 right through into the region of the " rectal glands." Here I was stiuck by 

 the immense masses of supporting tissue (^st.) lying under the raised rectal 

 epithelium. It seems probable that this tissue occurs in the recta of all 



