130 THE VOYAGE OF H.M.S. CHALLENGER. 



The second line, leading to Perophora, shows more modification. A stolon like that 



of Clavelina, but usually longer and more regular, has been formed, and permanent 



colonies are produced. A change has also taken place in the shape of the body and in 



the relations of the alimentary canal. The branchial sac has become relatively larger, 



both antero-posteriorly and dorso-ventrally, the result being that it occupies nearlj- the 



whole length of the short wide body. The stomach and intestine, there being very 



little space posterior to the branchial sac, have come to lie upon its left side and dorsal 



edge, thus producing a short-bodied Ascidian with no " abdomen." This shortening of 



the antero-posterior extent of the body by the alimentary canal coming to lie alongside 



the branchial sac (Fig. 20), has apparently been produced independently in several 



distinct groups of the Tunicata, viz., Perophora, Ascidia, Corella, Botryllus, and the 



higher Simple Ascidians. 



Br. 

 At. 



Fig. 20. — Diagram sho%ving a modification of tlie ancestr.il Asciilian by wliicli t'ce boily is sliorteneil 

 anterO'iiosteriorly and the alimentary canal is placed alongside the brancliial sac. 



a. amis; At. atrial aperture; Br. branchial aperture: h^'.s. branchial sac; i. intestine; )n. posterior end of tlie 

 body by which it is attached and from whicli outgrowtlis forming buds are produced ; a. ffisophagus ; r. rectum ; 

 St. stomach. 



The third line, leading from the Protoascidiates (E. in table, p. 120), is the very 

 important main liranch which gave origin to the various groups of Simple and Compound 

 Ascidians. The ancestral forms occupying this line are most nearly represented at the 

 present day by Ecteinascidia, an interesting genus first made known as a result of the 

 Challenger investigations,^ and since that studied by Sluiter ; " but I am inclined to 

 believe that the branch (F. in table) leading to most of the groups of Compound 

 Ascidians was given off rather earlier than the twig leading to Ecteinascidia. 



The small aberrant group of the Hypobythinee ' was probably formed as a side liranch 

 between E. and F. (see table). Hypohythius agrees with the ancestral Clavelinids, and 



' Herdman, this Keport, Part I. p. 239. 



- Ueber einige oinfache Asoidiea v. d. Insel Billiton, Katuurlcund. Tijdschr. v. Xcdcrl. Jndii', DL xlv. p. IGO, 1886. 

 * Moseley, On two new forms of Deep-sea Ascidians, Trans. Linn. Soc. Loud., ser. 2 (Zool.), vol. i. p. 287, 1877 ; 

 and Herdman, this Report, Part I. p. 227. 



