The Worms and some of their Posterity 



upper communicates with the outside by means of a large number 

 of transverse shts on its sides. Water is continually being taken 

 in by the mouth and passed along this upper canal, to reach the 

 outside by way of the gill slits, and in doing so it passes over the 

 gills, where the blood is circulating in fine capillary vessels. 

 Here the blood is supplied with 

 oxygen from the water, and is at 

 the same time relieved of carbonic 

 oxide. This, it will be observed, 

 is the same method of respiration 

 as that of the fishes. Behind the 

 last gill slit the digestive canal 

 becomes a simple tube, with two 

 digestive glands or liver sacs. 

 There are two main blood vessels, 

 the larger running along above the 

 digestive canal, and the smaller 

 below it, the two being connected 

 by means of numerous branches. 

 There is a swelling of the dorsal 

 vessel — a heart — at its forward ex- 

 tremity, in the base of the pro- 

 boscis. The nervous system is 

 peculiar ; it consists of two nerve 

 cords, the smaller below the gut 

 and the larger above it — the latter 

 therefore occupying a position simi- 

 lar to that of the spinal cord in 

 the vertebrates. Thus in two 

 respects, in its respiratory and nervous systems, Balanoglossus 

 must be regarded as a highly extraordinary member of the 

 w^orm group, and in both its peculiarities it shows an approach 

 to the vertebrate. There can be little doubt as to the position 

 of this group as an important connecting link between the ordinary 

 worms and the vertebrates. Fig. 60 illustrates the main features 

 of the anatomy. 



77 



60. — Section of front end of 

 Balanoglossus. 



Snout ; ;;/, mouth ; h^ heart ; co, body 

 cavity ; d^ alimentary canal ; n^ n, nerve 

 cords ; vg, ventral blood vessel ; dg, 

 dorsal blood vessel ; f, fold dividing 

 the alimentary canal ; vd, food canal ; 

 k, gill slits. 



