REPORT ON THE TUNICATA. 143 



the colony became thicker and more massive (Polycyclus in the one group, and 

 Sarcohotrylloides in the other). 



Returning now to the main axis of the Simple Ascidians, it is found to acquire 

 the folded branchial sac soon after the separation of the ancestral Botryllidse, 

 and immediately after this, close to the point of division (M. in table) into 

 primitive Cynthinse and primitive Styelinse, the genus Bathyoncus was given 

 off as a side branch. The origin of this form may either have been (as shown 

 in table) from the main axis before it divided, or from the branch of the Styelinae 

 not far along from the point M. Bathyoncus ^ is a deep-sea genus, in which the 

 branchial sac has undergone that remarkable modification, already discussed (p. 133), 

 which results in the total suppression of the fine interstigmatic vessels. It is closely 

 allied in other respects to the primitive Styelinse, and has simple tentacles. This 

 peculiar modification of the branchial sac has not been met with in any of the true 

 Cynthinse, but it occurs in Culcolus and Ftingulus amongst the Bolteninse (see 

 table, Culeolus, and Fig. 28, p. 147). 



In the primitive Styelinse the tentacles remained simple as they were in the 

 ancestral Ascidite Simplices, and the number of folds in the branchial sac became 

 limited to four on each side. The genus Pelonaia, which was formerly regarded^ as a 

 very abnormal Tunicate, difiering in some important respects from all other Ascidians, 

 is now known ^ to be an ordinary Simple Ascidian, and it was probably derived from the 

 ancestral Styelinse (table, p. 150). The folds in the branchial sac have disappeared in 

 Pelonaia, but they are found in a more or less rudimentary condition in a number of 

 species of the genera Styela ^ and Polycarpa. 



Throughout the sub-family Styelinse comparatively little modification has taken 

 place. The tentacles never become branched, and the branchial sac remains in a simple 

 condition, with four folds on each side. It is very interesting to find that even in 

 those Styelinas where the branchial folds become lost (e.g. Styela oblonga), it is still 

 possible to make out indications of their former presence by means of the arrangement 

 of the internal longitudinal bars and the sizes of the meshes and stigmata. 



The reproductive organs consist of one or two long convoluted masses upon each side 

 of the body in typical species of Styela, and of a large number of small rounded masses, 

 called " polycarps," attached all over the inner surface of the mantle in the genus 

 Polycarpa ; but species are known which form a perfect series of gradations between 

 these two conditions.^ Probably both of them, as well as the form of genitalia found in 

 the family Botryllidaa, are derived from an ancestral condition, which would be found 



1 This Report, Part I., p. 1G5. = Sec Forbes and Hanley, IJrit. JIolI., vol. i. p. •!:?. 



3 >['Intosh, Ann. and Mag. Nat. Ilist., ser. 3, vol. six. p. -lU, 1»67 ; and Traustedt, Vid. Medd. Xat. For. 

 Kjpbcnhavn, 1879-80, p. 418. 



■• On Individual \'ariation among Ascidians, Proc. Lit. Phil. See. Lirerpoul, vol. xxxvi. p. 315, 1882. 

 = See Sluiter, Naluurkund. Tijdschr. v. Ncdcrl. India, Dl. xlv. pp. 188 and 228, 1886. 



