Watt. — On Boltenia pachydermatiua. 339 



Following oa the tentacles comes part of the mouth-region, 

 spoken of by Herdman as the prsebranchial zone, bounding 

 which on the posterior side are the two peripharyngeal bands. 

 These surround completely the prsebranchial zone, surround- 

 ing on the one side the dorsal tubercle, on the other adjoining 

 the oral end of the endostyle. Following upon the peri- 

 pharyngeal bands comes the wall of the branchial sac proper. 



B. Peripharyngeal Bands. — The two peripharyngeal bands, 

 as before stated, bound the prsebranchial zone on the posterior 

 side — i.e., on the side far removed from the tentacles. On the 

 one side these bands immediately surround the dorsal tu- 

 bercle, then run in a somewhat wavy manner around the 

 circlet of tentacles, and join the endostyle at its oral end. 

 The anterior band forms a complete ring ; the posterior is 

 complete only at the sides. At the anterior end its right and 

 left halves become immediately continuous with the mar- 

 ginal ridges of the endostyle ; at the posterior, they are 

 continued into the anterior end of the dorsal lamina. 



c. Branchial Sac. — The peripharyngeal bands bound the 

 commencement of the branchial sac. This organ extends from 

 the anterior to the posterior end of the body. Its ventral 

 edge is formed by the endostyle, which is attached to the 

 mantle along the whole ventral surface. Along the dorsal 

 edge the branchial sac is free between its attachment to the 

 body (immediately posterior to the peripharyngeal bands) and 

 the gullet. On the dorsal surface of the body, at the pos- 

 terior end, the branchial sac opens into the gullet. The 

 wall of the branchial sac is thrown into large folds, 

 projecting into the interior of the sac, and running from 

 the peripharyngeal bands to the neighbourhood of the oeso- 

 phageal aperture. These folds are directed towards the 

 dorsal lamina ; there are always six of them on each side. 



In the walls of the branchial sac are the following sets of 

 vessels: (1) transverse, (2) fine longitudinal, (3) internal longi- 

 tudinal, (4) large transverse. (1.) The transverse vessels run 

 at right-angles to the length of the sac. (2.) The fine longi- 

 tudinal are in the same plane as the transverse, but run 

 longitudinally, and serve as a means of communication be- 

 tween the transverse. (3.) The internal longitudinal vessels — 

 larger vessels than the transverse — occupy a plane internal to 

 them, and run at right-angles to them from the anterior to the 

 posterior end of the branchial sac ; they are connected with 

 the transverse by short wide connecting-ducts placed at the 

 points of intersection. The rectangular meshes visible to 

 the naked eye are formed by the latter vessels and the trans- 

 verse. (4.) External to the transverse vessels, and connected 

 with them by the fine longitudinal, are vessels of much stouter 

 calibre. These radiate from the dorsal free edge of the 



