Chap. III.] 



ENTER OPNE us TI. 



combine with them the knowledge of the fact that, in 

 certain Nemertines, the nerve cords, instead of lying 

 at the sides of the body, tend to take up a dorsal posi- 

 tion. Whether or no Hubrecht is right in regarding 

 the Nemertinea as giving us indications of where to 

 look for the ancestral form of the Chordata, it is 

 clear that we must sharply distinguish them from the 

 group of the Platyhelminthes, with which they have 



Fig. 42. A, Diagram to show the relation of the proboscis (pbs) to the 

 surface of the body and to the sheath of the proboscis (pbs), in the 

 Nemertinea; (B) diagram of Petromyzon (the lamprey) showing the 

 hypophysis cerebri (hyp) ; the chorda dorsalis (ch) ; the mouth (m) ; 

 and the anus (a). (After A. A. W. Hubrecht.) 



been hitherto very closely associated. Linens, Cari- 

 nella, Polia, are examples of this group. 



So, again, in another group of " worms," the 

 Euteropiieiisti, the sole representative of which is 

 the remarkable Balanoglossus (Fig. 43), the anterior 

 portion of the eiiteron divides into a ventral and a 

 dorsal portion ; the former retains its nutrient office, 

 but the latter has chitinous lamellae developed in its 

 walls ; between these clefts (br) appear, which finally 

 open on the surface of the body ; blood-vessels are richly 

 distributed to the walls of the arches, and the water 

 taken in by the mouth passes through the clefts to 

 the exterior. In Balanoglossus, therefore, just as 



