Evolution of Fishes from Invertebrates 67 



Fig. 2. Long. sect, head region of Geonemertes austraVts, m, 

 mouth ; co, common opening of mouth and proboscis tube ; m.p. 

 velum; d.c, v.c, dorsal and ventral brain masses; c.gl, cephalic 

 gland ; oes, i, 2, 3 anterior, mid, and posterior oesophageal areas, 

 the dark area of the mid-oesophagus is highly glandular, and seems 

 at least in part to be the primitive rudiment of the thyroid gland; 

 d. gut, intestinal diverticulum or probable hepatic process; ep. 

 epidermis; cm, l.m, circular and longit. muscle masses; d.gl, dorsal 

 glands; ps. muscular proboscis tissue. (Reduced from Dendy). 



A like description would apply to the mouth and buccal 

 region of Cyclostomata and other fishes. But the velum 

 or membrane deserves more detailed consideration. This 

 has been accepted above as the bounding edge of the united 

 proboscis and proboscis sheath of higher nemerteans. 

 Now if, as traced below, the proboscis becomes greatly 

 shortened, and at least in its hinder part converted into 

 the pituitary body or hypophysis of cvclostomes and other 

 fishes, at the same time that the proboscis-sheath becomes 

 the notochord, the velum would be a ridge or expanse in 

 front of the point where the notochord ends and the up- 

 curved pituitary body joins the infundibulum. Such is ex- 

 actly the relation as indicated by the subjoined figure (Fig. 

 3) when supplemented by others, given by Dohrn (Camb. 

 Zool. 391). Alike in Amphioxus, in Cyclostomes, and in 

 higher fishes, it can be traced, at times prolonged into lobes 

 or at times becoming richly vascular lateral thickenings, as 

 in the Dipnoi. Morphologically, as well as phylogenetical- 

 ly, therefore, the velum is a very persistent and important 

 demarcating ridge. 



