1892.] On the Development of the Stigmata in Ascidians. 505 



IV. "On the Development of the Stigmata in Ascidians." 

 By Walter Garstang, M.A.. Jesus College, Oxford; 

 Berkeley Fellow of the Owens College, Manchester. Com- 

 municated by A. Milnes Marshall, M.A., M.D., F.R.S. 

 Received April 14, 1892. 



The respiratory organ or pharynx of the Tunicata exhibits a great 

 amount of variability in form, size, and complexity of structure in the 

 different members of the group — a variability which is obviously cor- 

 related with the physiological value of the organ. In the simplest 

 forms, the Perennichordata or Appendicularians, the pharynx is a 

 mere hollow cylinder, provided with a single pair of tubular gill- 

 clefts, one on each side. In the higher forms (the Caducichordata) 

 the cavities of the two gill-clefts become enormously dilated, so as to 

 constitute a pair of peribranchial chambers interposed on either side 

 between the pharynx and the body- walls. The dilatation of the gill- 

 clefts to form peribranchial chambers can be aptly compared with the 

 formation of branchial pouches in the tubular gill-clefts of Marsipo- 

 branch Fishes. But whereas the respiratory surface of the branchial 

 pouches of Marsipobranchs is increased by folds of the walls of the 

 pouches, the same purpose in the higher Tunicata is effected by a 

 different means. The inner (visceral) walls of the peribranchial 

 chambers apply themselves closely to the wall of the pharynx, and 

 perforations then appear at numerous points, where the pharyngeal 

 and peribranchial membranes have actually united. The remnants 

 of the pseudo-ccelomic cavity, enclosed between the pharynx and the 

 visceral walls of the peribranchial chambers, become extensive 

 channels for a vigorous circulation of the haemal fluid. The organ 

 formed by the union of the pharyngeal and peribranchial walls is 

 usually referred to as the " branchial sac : " and the perforations, which 

 put the cavity of the pharynx into extensive communication with the 

 peribranchial chambers, constitute the so-called " stigmata." 



The stigmata vary much in form and arrangement. In the fixed 

 Ascidians, whether simple or compound, they are usually simple slits, 

 of a narrow elongated form, arranged in a series of rows placed 

 transversely to the longitudinal axis of the body (Ascidia, Clavelina, 

 BotryUus, Styela). In some genera, however (Corella, Molgula), the 

 stigmata are curved and somewhat spirally arranged ; but this con- 

 dition is undoubtedly derived from the former by modifications of a 

 secondary nature. In the pelagic Tunicata (Salpa, Doliolum, Anchinia, 

 Pyrosoma), the condition met with in the fixed Ascidians is never 

 found ; there is never more than one row of stigmata on each side, 

 and this row, though occasionally oblique or even transverse, is 



