100 WILLIAM BATESON. 



structure of Stage H is of course shown by their ontogeny. 

 They are derived from it chiefly by increase in size of the 

 praeoral lobe, change in direction of the mouth, growth of a 

 rudimentary operculum, serial repetition of the gill-slits, and 

 appearance of the generative organs also as a serial repetition. 

 That any animal possessing a large praeoral lobe should 

 acquire a thick sheath of nervous tissue (especially when con- 

 sisting of fibres for the most part) is easily understood. As 

 shown in the foregoing pages, this mass of tissue is probably 

 mainly composed of aflferent fibres connecting the proboscis 

 with the dorsal cord. As soon as the ventral uerve-cord arose 

 as a concentration of nerve-tissue, this would naturally be 

 followed by another circular concentration in the nervous 

 sheath connecting the ventral cord with the central, invagi- 

 nated, nervous system, also as an afferent mechanism. 



In all probability the enormous increase in size of the larger 

 species was a comparatively recently acquired feature, as also 

 the peculiar odours which they emit ; to this latter power it is 

 possibly not too much to attribute the preservation of such a 

 group. 



Of the Cephalochorda. — The relations of the Cephalo- 

 chorda is the next subject for consideration. 



The young Balanoglossus agrees with Amphioxus, especi- 

 ally in the following anatomical features : — 



(1) The digging mouth. 



(2) The repetition and folding of the gill-slits. 



(3) The repetition of the generative organs. 



(4) The peculiar fate and remarkable asymmetry of the 

 anterior mesoblastic pouch and proboscis pore. 



(5) The presence of atrial folds. 



(6) The absence of (a) any developed sense organs; (b) any 

 excretory glands differentiated as such. 



(7) In the presence of excretory tubes opening into the 

 atrial cavity. 



On the other hand it differs from it in — 

 (1) The relative size of the praeoral lobe. 

 (3) The degree of its mesoblastic repetition. 



