770 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



and tho Marsipobranchs tlio repetition is not regular and opposite. 

 The gill-slits are, by their nature, repeated structures, but as they, 

 and a fortiori their repetition, arose within the limits of the Chordate 

 group this fact does not weigh against Mr. Bateson's theory that the 

 primitive memhers of it were unsegmcnted. 



The author next discusses the Entcropneusta as members of the 

 Chordata, and points out that the three features which distinguish 

 the Chordati from other animals are present in Balanoglossus ; the 

 central nervous system arises by longitudinal delamination from the 

 skin in the dorsal middle line, there is a hypoplastic notochord at 

 the anterior end, and the gill-slits are formed as regular fusions and 

 perforations of the body-wall and gut from before backwards. Mr. 

 Bateson objects to the suggestion that the lancelct and the Marsipo- 

 branchs are degraded forms, and points out that in Ascidians tho 

 change, though well-marked, is not a deviation from a segmented to a 

 less segmented form. 



It seems certain that the primitive Chordata were of aquatic 

 habit, led a free life, and possibly burrowed ; the mouth had primi- 

 tively a ventral direction, the skin was ciliated ; the nervous system 

 of Balanoglossus, AmpMoxus, the lampreys, and Myzine form a gradu- 

 ated series leading up to the conditions found in higher vertebrates, 

 and showing the evolution of the system from a solid cord in the 

 skin to a close tube whose walls give off a series of "segmental" 

 nerves. : 



As to the affinities of the Chordata, we can only say, as regards 

 the Echinodermata, that Tornaria is practically identical with Bipin- 

 naria ; it is still a matter for inquiry whether the resemblances to the 

 Nemertinea are indications of genetic affinity. The relations of the 

 Chordata inter se may be shown by the following table : — 



Cyclostomata 



Protognathostomata 



| (of Balfour) 



Enteropncusta 



? Nemertines 



Cephalochordata 



Tunicata 



Form with one gill-slit 



