REPORT ON THE TUNICATA. 125 



Taking the first of these lines (in table, p. 120), it is found that the organisms which 

 diverged towards the Thaliacea from the ancestral Appcndiculariidaj allied to Moss' form 

 remained free-swimming, but acquired the power of passing currents of water through 

 their respiratory systems of cavities (the branchial sac, and the peribranchial cavity 

 formed by the union posteriorly and dorsally of the two ventral tubes leading in the 

 Appendiculariidse from the stigmata to the exterior) in such a way as to propel them- 

 selves through the water. This would naturally result in the great enlargement 

 of these cavities, and in the arrangement of the muscle fibres of the body-wall in 

 a series of transversely placed bands, which would serve to drive out with force the 

 contained water. The posterior region of the body, or tail, being now superseded as 

 an organ of locomotion, would become gradually suppressed, and thus the ancestral 

 Thaliacea would be evolved. 



From these Protothaliacea, in which also reproduction by gemmation from a stolon 

 and consequently alternation of generations became established, two diverging lines 

 lead to the Salpidaj and to the Doliolidae. The test or outer tunic, that remarkable 

 structure so characteristic of the group, was probably first formed in the ancestral 

 Appendiculariidse, since it is found represented at the present day not only in the 

 Ascidiacea, l)ut also in the Thaliacea and in the Larvacea. Probably at first it was 

 merely a temporary cuticular secretion of the ectoderm formed as a protection during 

 some particular period, and comparable with the "Haus" now produced l)y some 

 members of the Appendiculariidse. It afterwards, however, became converted into 

 a permanent layer of considerable thickness covering the outer surface of the body, 

 and then finally became organised by the migration into it of ectoderm cells which 

 proliferated and became modified in various ways, to form the complicated test 

 structures found in many Tunicata. 



In the line of ancestral forms leading to the Doliolidai, however, the test must have 

 remained in a very slightly developed condition, or may even have become more 

 rudimentary, since in the Doliolidae as now known the test is almost absent, being 

 merely represented by a very delicate film covering the ectoderm. The Doliolidae are 

 much less modified than the Salpidse, they still retain a " tailed " stage in their life 

 history, and their branchial sac diflers comparatively little from that of Appendiculana 

 mossi, and therefore we may consider that the Doliolidas (see Fig. 15) represent more 

 nearly than do any of the Salpidse the essential structure of the Protothaliacea. 



In these ancestral forms the transverse muscles were probably difi'use, and scattered 

 irregularly through the mantle ; the branchial sac would be in much the same condition 

 as in Appendicularia mossi, but the atrial aperture, the common excretory opening 

 leadino- from the peribranchial cavity, had come to be placed at the posterior end of 

 the body so as to 1)e directly opposite to the branchial aperture, and so allow the water 

 to pass straight through the body (see Figs. 15 and 16). 



