324 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D s er . 



The hypophysis is a compound tubular gland with eight 

 or nine tubules emptying into the duct. On account of the 

 thinness of the mantle in this region they are all in the same 

 plane. Anteriorly a corner of the gland is ventral to the 

 ganglion, but posteriorly it is at the same level or dorsal to 

 the latter. 



The ganglion usually has a pair of anterior nerves, a 

 latero-anterior pair, and five irregularly arranged ones 

 extending from its posterior end. Variations are, however, 

 quite frequent. 



The Branchial Sac. 



The branchial sac is attached to the mantle along the 

 extent of the peripharyngeal band, epibranchial groove, 

 endostyle, and postbranchial groove, and to the intestine 

 along the whole of its terminal part, which is dorsal to the 

 branchial sac. Many connectives extend to both mantle 

 and viscera, but they have no very definite arrangement on 

 account of the irregularity of the vessels. 



The arrangement of the vessels is subject to considerable 

 variation, a large part of which, however, depends upon 

 the age of the individual. Figure 12 shows a portion of 

 the branchial sac of the smallest individual collected. The 

 conditions here are very regular and simple when compared 

 with the older ones; but even in these a careful study 

 shows essentially the same structure. There are four 

 kinds of vessels: (1) The outer vessels (o, fig. 12), 

 which occupy the outer surface of the sac and unite with 

 all the connectives. They extend in longitudinal, trans- 

 verse, and intermediate directions, and bound the spaces in 

 which the spiral stigmata are contained. They are quite 

 conspicuous, especially when seen from without, and con- 

 tain most of the blood. (2) The spiral interstigmatic 

 vessels, which, together with the outer ones surrounding 

 them, limit the stigmata. In the younger individuals these 

 are in a plane but slightly interior to the outer vessels, but 

 later an increase in surface is effected by drawing the 



