128 



THE VOYAGE OF H.M.S. CHALLENGER. 



point the animals changed their habits and became fixed. And just as the tailed larval 

 Ascidian at the present day after a short free-swimming existence becomes attached, 

 loses its locomotory organ, and undergoes other changes, so the ancestral Ascidian when 

 it settled down on some object to lead a stationary existence, probably went through a 

 similar but more gradual process of degeneration. The tail with its contained notochord 

 and muscles being no longer necessary, would become rudimentary and disappear. The 

 well-developed sense organs, which were most important to a locomotory organism, 

 became almost useless through the change of life, and they also were suppressed, and, as 

 a result, the nervous system which had l)een in most intimate connection with the sense 

 organs underwent considerable degeneration. On the other hand, the alimentary canal, 

 and especially its respiratory portion, or branchial sac, became greatly enlarged and 

 somewhat more complicated. A w^ell-marked permanent test was also produced, and the 

 atrial aperture — formed by the union of the two primitive laterally-placed peribranchial 

 openings — came to lie on the dorsal surface of the body not far from the anterior end. 

 In this way an ancestral form (E. in taltle, p. 120) was produced somewhat resembling 

 many of the Simple Ascidians, and probably more like a solitary Clavelina than any 

 other existing form' (Fig. 18). 



..Br. 



Fig. is.— Diagram of one of the hy|iotlietical Protoascidiaoea, showing what is probably the 

 primitive condition of the Ascidian Alimentary Canal. 



a, anus; At. atrial apertnre : Br. branchial aperture; hr.s. branchial sac; i. intestine; m. posterior end of the 

 body by which it is attached and from which outgrowths forming buds are produced ; cc. oesophagus ; r. rectum ; 

 St. stomach. 



Probably this form, or one of its immediate ancestors, acquired the power of repro- 

 ducing by gemmation, so as to form small, and possibly at first only temporary, 



* In 1882 (this Report, Part I. p. 285) I placed Clavelina close to the ancestral form of the Ascidire Simplices. 

 Kecently Van Beueden and Julin have come to the same conclusion as the result of their embryological investigations, 

 and they regard Clavelina as the most archaic of Ascidians (Arch, de Biol, torn. vi. p. 327). 



