63 



which pass thi()Ui;ii tlic sac to the sinus the epithelium is columnar and secretory. 

 Tlic wall (if the nephridial sinus is formed of three coats: the nephridial 

 epithelium : the vascular endothelium ; and an intervening sheet of connective 

 tissue which contains arteries and capillai-ies , the vasa vasorum. This wall is 

 evaginated and invaginated to foim a series of short intei'digitating tubes. The 

 blood tills the tubes which open into the sinus and the nephridial excretion is 

 discharged into those which open into the nephridial sac. The sinus is divided 

 by a number of constrictions, four or six, into several lobes and the veins coming 

 from the pancreas and the rectum form four small additional lobes. 



The Resp(ratory Circulation. 



Each branchial heart (hr. h.) is biscuit-shaped and lies within its pericardium 

 just under the integument and at the level of the dorsal half of the systemic 

 heart. It bears upon its inner edge a small gland, the "pericardial gland", which 

 has a small irregular lumen opening into the pericardium. The epithelium of the 

 lumen and of the surface of the gland is columnar : that of the portion of the 

 heart around the gland is cubical , and that upon the remainder of the heart pave- 

 mental. The heart is composed of striated muscle-flbres and spherical or ovate 

 cells w'hich are apparently secretory though their function as well as that of the 

 pericardial gland is unknown. The tissues of the branchial heart are supplied 

 with arterial blood by the posterior aorta. The venous blood from the nephridial 

 sinus enters the ventro-internal portion of the heart through an opening guarded 

 by a pair of semi-lunar valves and passes out into the branchial artery through 

 an opening at the ventro-external edge of the heart. The base of the artery is 

 enlarged and has strong circular muscle-fibres. Four tubercles, each typically as 

 long as the diameter of the artery , project into the lumen in the direction of 

 the current. Although four tubercles are always present, one, less commonly two' 

 or rarely three, are small or minute while the others are correspondingly enlarged. 

 The contraction of the circular muscle at the base of the artery presses together 

 these tubercles and closes the vessel preventing the flow of blood in either direc- 

 tion. This valve makes it very difficult to obtain successful injections of the gill. 

 The branchial artery passes through the center of the gill giving off throughout 

 its course small alternating branches. 



The Gill. The gill has been the subject of an extended anatomical and 

 embryological paper by Joubin, and we find that his conclusions hold for this 

 species. The gill appears early as a small papilla projecting from the posterior 

 -Side of the embryo in the area between the mantle folds, the siphonal folds and 



