Paekee. — Anatomy and Embryology of Scymnus lichia. 229 



In front of the mid-brain comes the greatly elongated thalamencephalon 

 or 'twixt-brain (the) ; it is best described as a shallow trough, roofed over 

 only by a small band of nervous matter at its hinder end and for the rest 

 by pia mater ; the third venticle or thalamoccele is thus widely open above, 

 and there are no lateral thickenings answering to thalami optici. 



In most Elasmobranchs, — indeed, according to Gimther, in all, — there 

 exist on the ventral surface of the thalamencephalon just behind the optic 

 chiasma (o.c), paired ovoidal bodies, the lobi inferiores : there is no trace of 

 them in Scymnus ; the thalamencephalon is merely produced ventrally into 

 a thin -walled tubular infundibulum (inf), which extends backwards over the 

 ventral surface of the mid-brain and is continued directly into the hollow 

 thin-walled pituitary body (pty). Extending along the middle ventral line 

 of the infundibulum and pituitary body is a flattened one-lobed saccus 

 vasculosus (s.v). 



The prosencephalon is very interesting : instead of forming a transversely 

 elongated mass, either solid, as in Raja, or containing small ventricles, as in 

 Scyllkim, it consists of a small unpaired hinder portion (pre) continuous 

 with and passing insensibly into the thalamencephalon, and of paired, 

 divergent, pyriform bodies, the cerebral hemisphere (c. h). Similarly, the 

 cavity of the prosencephalon consists of an unpaired posterior portion (pre) 

 which may be conveniently called the prosocoele, and is perfectly continuous 

 with the third ventricle, and of paired lateral ventricles (I. v). The walls 

 of the whole fore-brain are very thin, and there is no constriction between 

 the lateral ventricles and the prosocoele, or between the prosoccele and the 

 third ventricle. 



The olfactory lobes (olf) are comparatively short, dilated at their ends, 

 and contain olfactory ventricles (olf. v) continuous with the lateral ven- 

 tricles. 



The* brain of Scymnus is thus seen to exemplify with diagrammatic 

 clearness the typical structure of the vertebrate encephalon. We have the 

 large fourth ventricle ; the cerebellum retaining its primitive character of a 

 hollow out-pushing of the roof of the fourth ventricle ; the mid-ventricle 

 showing no distinction into aqueduct of Sylvius and optic ventricles, and of 

 approximately equal calibre with the third and fourth ventricles ; the prosen- 

 cephalon, or cerebral rudiment of the embryo, composed of an unpaired 

 hinder portion which bifurcates in front to form the paired hemispheres, and 

 these again continued insensibly into the olfactory lobes ; the fore-ventricle 

 or cavity of the fore-brain, in the form of a Y-shaped space, the stem of the 

 Y being represented by the prosocoele and third ventricle, the arms by the 

 lateral and olfactory ventricles ; finally all the cavities are large, and their 

 walls but little thickened : this is especially noticeable in the case of the 



