540 



Causes and Course of Organic Evolution 



part of the proboscis attachment, and the entire area round 

 the proboscis-sheath, have here become the collar. The an- 

 terior expansion in front of the notochord and inside the pro- 



d^- ^pg-, p.s. 



op. 



c.c. '-ug. tc. 



Fig. 26 — Comparative diagrammatic views of cephalic region of, a nemer- 

 tean, h nemerto-hemichord type, c protohemichord type, d Balanoglossus. m. 

 mouth, i. p. everting inner glandular proboscis part in a, b, that becomes p. c. 

 or proboscis in c-d, o. p. outer or muscular part of proboscis, v. p. g. ventral 

 proboscis glands, p. s. proboscis sheath becoming notochord nc in d, d. g, v. g 

 dorsal and ventral brain masses connected by transverse commissure tr. c, d. 

 n., V. n. dorsal and ventral nerves, p. c. proboscis cavity, c. c. collar cavities, 

 t. c. trunk cavities, a. np., p. np. anterior and posterior neuropores. 



boscis known as the "proboscis coelom" or "the anterior body 

 cavity" would then represent a part of the nemertean body- 

 cavity, that by joint extrusion of the proboscis and forward 



