138 



THE VOYAGE OF H.M.S. CHALLENGER 



between Pyrosoma and the Compound Ascidians ; and Codocormus certainly in the 

 condition of the colony forms a link between Pyrosoma and such a genus as Distaplia. 

 Pyrosoma, then, must be regarded as a highly modified form derived from the ancestral 

 Didemnidse, and much more closely allied to the ordinary Compound Ascidians, such as 

 the Distomidse and the Polyclinidse, than to the other pelagic Tunicates, such as a 

 colony of Salpas. 



In order to trace the evolution of the remainder of the Simple and Compound 

 Ascidians,* it is necessary to return to the ancestral Ascidians allied to Ecteinascidia 

 from which the line F. was derived. In these forms gemmation took place from the 

 posterior end of the body, resulting in the formation of small colonies, possibly temporary 

 only, in which, however, the Ascidiozooids were quite distinct, and were not embedded 

 in a common test. The alimentary canal extended behind the branchial sac posteriorly, 

 and the heart and the reproductive organs were placed alongside the intestinal loop. 

 The branchial sac was provided with internal longitudinal bars, and the tentacles 

 were simple. 



From this -point Ecteinascidia (see table, p. 150) was derived, and shortly afterwards 

 the power of reproducing by gemmation so as to form colonies must have been gradu- 

 ally lost, since it is not found in the next group of Simple Ascidians, the Ascidiidse. 



Fig. 24.— a. specimen of Ascidia aspcrsa; B. specimen of Ciona iniestinalis, showing rudimentary stolons. 



Probably the common species Ciona intcstinalis is the nearest form known to the 

 primitive Ascidiidse. It resembles Ecteinascidia in most respects, but it is not 

 pedunculated, and has not the power of reproducing by gemmation. In many 

 specimens of Ciona, however, processes of the test containing blood-vessels are found 

 attached to the posterior end of the body (Fig. 24, B). These closely resemble the stolons 

 of the Clavelinidae both in appearance and in structure, and there can be no doubt 



' For further particulars in regard to the structure and relations of the Compound Ascidians discussed above, see 

 Part II. of this Report. 



