224 



AVERS AXD JACKSOX. 



EXPLANATION OF PLATE XXIL 



Fig. I. A cross-section of the notochord, spinal cord, and sheaths of 

 Bdellostoma a short distance behind the cranial region (x lo, camera lucida 

 outlines). 



Fig. 2. A cross-section of the notochord. spinal cord, and sheaths of 

 Bdellostoma in the posterior gill region (x lo, camera lucida outlines). 



Fig. 3. Section of the notochord and its envelope. 



Fig. 4. A cross-section of the cranium of BdeUostoma through the region of 

 the auditor}' capsules (x 20, camera lucida outUnes). 



Fig. 5. Dorsal %-ievr of the skull of Bdellostoma (x 2). 



Fig. 6. The skull and the skeleton of the pharj-ngeal region of Bdellostoma, 

 lateral \-iew. The roof of the spinal canal, the oesophagus, the gills and gill 

 passages, and the retractor mandibuli muscle are outlined with dotted lines ( x 2). 



= internal process of lateral 

 labial cartilage. 



= notochord. 



= nucleus. 



= notochordal sheath. 



= subnasal bar. 



= nasal tube. 



= oesophagus. 



= oesophago-cutaneous duct. 



= olfactory capsule. 



— pter}go-quadrate. 



= palatine bar. 



= parachordal cartilage. 



= supra-pharj-ngeal plate. 



= neural tube. 



= basal process. 



= skeletogenous layer. 



= internal layer of notochordal 

 sheath. 



= middle layer of notochordal 

 sheath. 



= external layer of notochordal 

 sheath. 



= spinal cord. 



. /.3, t.i = 1st, 2d, 3d, 4th tentac- 

 ular cartilages. 



= main bar of the trabecula. 



= anterior horn of the same. 



= tendon of retractor mandibuli 

 muscle. 



= vacuole. 



= external lateral velar bar. 



= internal lateral velar bar. 

 3, 4 = fenestrae of skull. 



