326 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Sek. 



As a result of all these processes, the branchial sac in 

 the older individuals is so complex and the regions so 

 different, even in the same animal, that if there were no 

 earlier stages to refer to it would be exceedingly difficult to 

 arrive at sound conclusions concerning the typical relations 

 of the various vessels. 



The endostyle, in the preserved specimens, is near the 

 extreme left edge of the branchial sac; but it is probable 

 that in life it was somewhat closer to the sagittal plane, 

 since the more powerful muscles of the left side would 

 draw it away from its natural position. Its posterior end is 

 continuous with the postbranchial groove, the most promi- 

 nent edge of which connects with the right edge of the 

 endostyle. 



The dorsal lamina is represented by a series of languettes 

 which vary from eleven to thirty-two in number. As a 

 rule they are more numerous in the larger individuals. 

 They extend from the posterior end of the epibranchial 

 groove to the edge of the smooth area surrounding the 

 oesophageal orifice, and their concave side is turned towards 

 the right. The great dorsal blood-vessel usually accom- 

 panying the lamina runs in the mesentery of the intestine 

 as far as the anus, and thence extends forwards, branching 

 very irregularly. Two strong bands of smooth muscle 

 fibres are contained in the lateral walls of this vessel. 



The Viscera. 



In discussing the position of the viscera, the sagittal 

 plane is best taken as passing through the two siphons and 

 dividing the animal as nearly as possible into symmetrical 

 halves. This will divide the disk into approximately equal 

 parts, except in the few cases where the siphons are con- 

 siderably nearer the right side, as in Traustedt's (1887) 

 fig. 2. In such cases, however, the plane as defined will 

 be more nearly the plane of symmetry of the whole animal 

 than if it were taken as cutting the disk midway between 

 the two sides and not including the siphons. 



