172 MR J. D. MACDONALD ON THE REPRESENTATIVE RELATIONSHIPS 



though finely areolated and thrown into parallel folds ; whereas, in all the re- 

 maining fixed Tunicata, a branchial sac may be demonstrated, the respiratory 

 slits or meshes being in general longitudinal, and disposed in many transverse 

 series. On the other hand, the respiratory system in the free or Pelagic section 

 presents no less than four distinct types, as occurring respectively in the genera 

 Pyrosoma, Doliolum, Salpa, and Appendicular ia. These may be defined as 

 follows : — 



Branchial membrane sac-like, rvith transverse slits in single longitudinal series, 

 strengthened by longitudinal non-ciliated bars. Apertures terminal or subter- 

 minal — P yrosom a . 



Respiring by an upper and a lower " gill-band,'''' connected with each other later- 

 ally, and with the walls of the atrium ; having branchial slits, but no suppo7'ti?ig 

 longitudinal bars. Apertures terminal — Doliolum. 



Respiring by a central inferior gill-band, with free borders and transverse 

 ciliated stripes, but without slits or bars. Apertures terminal or subterininal — 

 Salpa, &c. 



Pharynx ciliated below without a distinct " gill-band f branchial slits reduced to 

 two ciliated oj)enings on the sides of the rectum. Apertures approximated hoemally 

 — Appendicular! A . 



As far as the respiratory system is concerned, therefore, the fixed Tunicata 

 exhibit at least two well-marked types, and the Pelagic group four, which are 

 equally distinct, and, as I conceive, of equal importance, demanding fair consider- 

 ation in systematic arrangement. Moreover, I am quite satisfied that there are very 

 striking representative relationships existing between the fixed and free Tuni- 

 cata ; and in order to exhibit these the more clearly, I have drawn up the annexed 

 circular scheme or table, in which it will be seen that each subclass has its simple, 

 social, and compound groups, and their mutual representatives may be at once 

 recognised. Thus, Appendicidaria represents the equally curious genus Pelonaia, 

 Doliolum the remaining simple Tunicata, Salpa the social, and Pyrosoma, the com- 

 pound group, but, in particular, the Botryllians. Any one accustomed to draw 

 comparisons and analogies will readily perceive that there is something more 

 than simple coincidence in all this, particularly as the characters employed are 

 so comprehensive, bearing successively on habit of life, habit of body, social habit, 

 mode of gemmation, and respiration. It would be quite as easy to draw up two 

 circles as one, or the characters might be thrown into rays instead of circles, but 

 the result in all cases would be virtually the same. 



I always pay respect to the maxim, that " characters can only be taken as 

 natural when they have been proved to be so ;" and I know also that groups 

 must be found, before trustworthy characters can be chosen to effect a classifica- 

 tion. When we fiudiNerites breathing air, and feeding on the green leaves of the 



