SAHGENT: THE OI'TIC KEFLEX APPARATUS OF VERTEBRATES. 151 



regiuii of tlie vagus lobes has a greater width than any other portion of 

 the brain except tlie olfactory lobes. The sinus rhomboidalis is a wide 

 open space covered by the greatly convoluted and vascular plexus 

 chorioideus III (Fig. A). The cerebellum is but .slightly developed as 

 an independent structure. The optic lobes are large and nearly spheri- 

 cal. Along the median plane they are partly separated from each other 

 by a slit-like sinus covered by the greatly convoluted plexus chorioideus 

 XL The diencephalon is well developed and more conspicuous than in 

 any other vertebrate brain. It consists of the two lateral elongated 

 lobes of the optic thalamus separated dorsally by an open sinus, which 

 is covered by tlie plexus chorioideus I, an almost simple membrane. 

 The right ganglion habeuula is an almond-shaped lobe, so greatly devel- 



FiGURE A. Petromyzon marinus, adult. Nearly nitHlian parasagittal section of 

 the brain, showing Reissner's fibre projected on the plane of section. For 

 meaning of abbreviations, see Explanation of Plates, p. 267. 



oped as to lie mostly in the median plane (Plate 1, Fig. 4). The left 

 ganglion habenula is much smaller and lies on the left side, partly cov- 

 ered by the right ganglion. The prosencephalon consists principally of 

 two enormous olfactory lobes. 



The cyclostome brain has the lateral halves of its roof less completely 

 connected by nervous structures than that of any other vertebrate. A 

 sagittal section of the brain shows the roof largely vascular or mem- 

 branous. The only nervous structures in the median plane are the 

 small and primitive cerebellum, the posterior portion of the mesencepha- 

 lon, the posterior commissure, and the right ganglion habenula. The 

 essentially non-nervous character of the median brain-roof and its al- 

 most complete division into bilateral halves are the conspicuous features. 

 The plexus chorioideus II is represented iu guathostomes by the thin 



