4 ARTHUR DENDY. 



the adult, while iiiGeotria and Petromyzon it is the right 

 (posterior) member of the pair which becomes domiuaut. 



In Geotria the posterior (right) parietal sense-organ is 

 seen to be connected with an opaque-looking band of tissue 

 (fig. 1, P.S.), which runs backwards to the posterior com- 

 missure. This is the pineal stalk, including the pineal nerve 

 (cf. fig. 6), and representing the hinder part of the original 

 outgrowth of the brain, whose anterior part forms the " pineal 

 eye." Owing, doubtless, to the enormous development of the 

 right habenular ganglion (fig. 1, G.H.R.), the pineal stalk is 

 pushed somewhat to the left side. Posteriorly, the pineal 

 nerve is connected, as will be shown later on, both Avith the 

 right habenular ganglion and with the posterior commissure. 

 The left habenular ganglion is, as in Petromyzon, very 

 much smaller than the right, and is divided into anterior 

 and posterior portions, ^'he posterior portion (fig. 2, G.H.L.) 

 lies in immediate contact with the right habenular ganglion, 

 with which it is connected by a transverse band of fibres, 

 the commissura habenularis superior of Studnicka^s 

 terminology. The anterior portion (fig. 2, G.H.A.) lies imme- 

 diately beneath the left (anterior) parietal sense organ (para- 

 pineal organ), and is connected with the posterior portion by 

 means of a stout band of nerve-cells and fibres underlying 

 the pineal stalk and constituting the tractus habenularis 

 of Studnicka (fig. 2, T,H.). (In fig. 2 both right and left 

 ganglia habenulse are shown for diagrammatic purposes, 

 but it would not really be possible to see both in a strictly 

 median sagittal section such as is supposed to be represented.) 

 Almost immediately behind the right habenular ganglion, 

 but separated from it by a well-marked recess (the recessus 

 inf rapinealis), lies the posterior commissure (figs. 1 and 

 2, C.P.). In a longitudinal section of the brain (fig. 2) the 

 posterior commissure is cut transversely, and appears as a 

 somewhat oval body projecting downwards and backwards 

 into the brain-cavity at the posterior dorsal limit of the third 

 ventricle. On the antero-ventral face of the posterior com- 

 missure lies a conspicuous longitudinal groove (fig. 2, Ep.G.) 



