506 Mr. W. Garstang. On the [May 19, 



usually longitudinal in direction. In Pyrosoma and some species of 

 Doliolum the stigmata are narrow, elongated slits, extending trans- 

 versely across the whole lateral face of the pharynx — each stigma 

 occupying an area which in the fixed Ascidians is taken up by an 

 entire transverse row of stigmata. 



To ascertain what is the fundamental order underlying all these 

 variations, and to determine what degree of correspondence and 

 homology there is between the stigmata of the pelagic Tunicata and 

 those of the fixed Ascidians, is not an easy matter ; indeed, the possi- 

 bility of any detailed comparison hardly seems to have occurred to 

 the majority of investigators. Two views, due to Herdman and 

 Lahille respectively, are, however, worthy of mention here. 



Professor Herdman* derives Pyrosoma from a group of the com- 

 pound Ascidians, through the curious colonial form Coelocormus 

 Huxleyi ; and, in harmony with this view, he regards each of the 

 transverse stigmata of Pyrosoma as corresponding to an entire trans- 

 verse row of stigmata in the Ascidians, the several stigmata of the 

 row having apparently coalesced to form the single stigma of Pyrosoma 

 — a process which has almost certainly occurred in certain deep-sea 

 types (Fungulus, Culeolus, &c.). 



Lahillef characterises the latter portion of Herdman's view as a 

 " profound error," and attempts, instead, to establish the remarkable 

 proposition that the longitudinal row of transverse stigmata in 

 Pyrosoma is strictly homologous with one of the transverse rows of 

 longitudinal stigmata in an Ascidian, through a phylogenetic rotation 

 of position. The oblique position of the row of stigmata in some 

 species of Doliolum (e.g., D. Dhrenbergi) is regarded as an intermediate 

 condition between the two extremes. Lahille bases this proposition 

 upon the changes of position which the organs of a Pyrosoma-bud 

 undergo during development. These changes, it is true, are very 

 remarkable, but they furnish absolutely no evidence for Lahille's 

 contention ; for it is a well-established fact J that immediately after 

 their first appearance the stigmata of Pyrosoma begin to elongate in 

 a direction at right angles to the long axis of the endostyle, and this 

 relation is maintained through all the curious changes of form which 

 the bud undergoes in its further development. Lahille's homologies 

 are consequently without foundation ; and, although Herdman's com- 

 parison is far more justifiable, yet the development of the transverse 



* < " Challenger " Reports, 5 "Tunicata," 2nd Report, pp. 319, 320; 3rd Report, 

 pp. 20, 24, 25, 137. 



f ' Recherches sur les Tuniciers,' Toulouse, 1890, pp. 59, 61, figs. 44 — 51. 



X Seeliger : " Zur Entwickelungsgeschichte cler Pyro somen ;" ' Jenaisclie Zeit- 

 schrift,' vol. 23, 1889, p. 622, figs. 15, 17, 19. Salensky : " Beitr. z. Entwick. d. 

 P vro somen ;" ' Zoolog. Jahrbuch., Abth. f. Anat.,' vol. 5, 1891, p. 32. Salensky's 

 figures 2 and 3, on p. 9, are, however, strangely inaccurate in this respect. 



