KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 37 



outer sacs have their walls slightly but densely lobate especially in front, outwardly 

 and inferiorly. On their back sides the foot retractors run, at their exit, below, 

 plunging into deep furrows of the sacs; above, the retractors are situated behind 

 the kidneys and outside the pericardial tubes. The nephroprocts appear somewhat 

 beneath and out of the nephrostomes and form, as usual, only simple pores in the 

 body wall, without any papillae or other specialization of the epithelium. 



The pericardium is open throughout, though compressed from front to behind, 

 narrowing downwards into two short lateral corners, from which the nephridia emerge. 

 An excretory epithelium corresponding to the pericardial gland in other Lamelli- 

 branchs covers the outside of the auricles to their whole extent. 



Respiratory organs (figs. 31, 32). The gills are of the folded or ribbed type. 

 Both the anterior and the posterior one is folded and the folds of the descending and 

 those of the reflected lamina are opposite each other. Each fold is composed of 28 — 30 

 filaments, and these are transversally connected by dense interfilamentar blood 

 vessels. Besides them there occur connections within one and the same fold between 

 the opposite filaments (intraplicatural connections). Even the single folds belonging 

 to the same lamella have their principal filaments joined to each other by means 

 of »connections interplicaturales» (Anthony). Further there exist junctions from 

 one lamella to the opposite one; these interfoliar connections are established be- 

 tween opposite principal filaments, which are united through their whole or part of 

 their length by a blood vessel, of which every alternate one is arterial and the 

 interjacent ones venous. The junctions between the venous vessels begin to appear 

 nearer to the branchial axis than do the arterial ones. The venous septa, however, 

 are of two kinds, higher and lower, which alternate, so that every fifth is a higher 

 one. There exist, consequently, interfoliar connections between all opposite prin- 

 cipal filaments, contrary to the conditions stated by Anthony for Chama iostoma, 

 namely that, »pour la branchie interne ces cloisons interfoliaires existent dans la 

 region moyenne de 1'organe tous les cinq plis». These connections seen by Anthony 

 undoubtedly correspond to the higher venous septa. 



Besides the connections mentioned above, we found another kind in the inter- 

 marginal veins, forming a communication between vena marginalis of the reflected 

 laminae and vena branchialis. These are limited to the higher venous septa, but 

 occur only below the point where vena branchialis enters into the axis of the gill. 

 They are few in number and seem to have no correlation to the folds, though there 

 are generally, as has been mentioned, four folds of each lamina between them. 



The posterior (exteriör) demi-branch has a descending lamina of considerably 

 reduced size and also differs from the anterior gill in having the venous vessels 

 serving both as interfoliar and intermarginal copnections. An account of their 

 appearance is given in the preceding pages, as well as of the arteriae that establish 

 exclusively interfoliar connections. 



