REPORT ON THE TUNJCATA. 141 



clijinge took place, however, ii Lrancli diverged from tlic main stem, and after consider- 

 able modification gave rise to the family of the Botryllidaj. In this long line of descent 

 (N. in table, p. 1 50) the lost property of reproducing by gemmation was apparently 

 regained, ami as a result colonies were produced once more. 



The Ascidiozo<ii(ls in the ancestral Botryllidse became completely emljedded in a 

 (iunnuou test, but they remained in a short-bodied condition, the alimentarj^ canal 

 being placed alongside the thorax. The Ascidiozooids also, as the result of gemma- 

 tion, became arranged in systems, and in each system all the atrial apertures have 

 come to open into a centrally-placed common cloacal cavity (Fig. 26). The test 



Fig. 20.— a colony of BUnillus. The AscidiozooiJs are arranged in eight systems. 



a. a large system formed of fifteen Ascidiozooids ; b. a sm.all system formed of seven Ascidiozooids ; c test ; 

 d. cloacal aperture ; c. an Ascidiozooid ; /. terminal knobs of vessels ; 'j. a fully expanded cloacal aperture. 



is penetrated in all directions liy a well-developed system of blood-vessels with 

 enlarged terminal bulbs in the superficial layer of the colony, forming an accessory 

 organ of respiration.^ This system is evidently the same as that found in the 

 test of some of the Ascidiidas, and has doubtless been inherited by the Botryllida; 

 from their ancestors amongst the Ascidise Simplices. The branchial sac in the 

 Botryllidse is well developed, and agrees with that of the Simple Ascidians from 

 the point F. (table, p. 150) onwards in having well-developed internal longi- 

 tudinal bars. The reproductive organs, finally, in the Botryllidse are found in a 

 condition which suggests the close connection with the ancestral CynthiidtB shown 

 in the table. 



One of the new Compound Ascidians discovered during the Challenger Expedition, 

 Sym2)Iegma viride, from Bermuda, is a remarkable form which unites the external 

 appearance and general arrangement of colony characteristic of the typical Distomidse, 

 with the structure of branchial sac and dorsal lamina found only in the Botryllidse 

 and the Simple Ascidians. In the second part of this Eeport (p. 144), while pointing 

 out the resemblance of Synvplegma to the Botryllidse, I placed the genus provision- 

 ally in the Distomidse ; but I am now inclined to regard it as being probably the 



' See Prov. Lit. and J'hU. Soc. Liverpool, vol. xxxix. p. 39, 1885. 



