68 WALTER GARSTANG, M.A. 



tubes of the abdominal walls in Clavelina producta. The only other 

 "social Ascidian" possessing morphologically similar structures is 

 Sluiteria rubricollis (Van Beneden* and Sluiter), whose transparent 

 test is traversed by several sterile stolonial tabes, branching 

 dichotomously and terminating in a few delicate papillary prolonga- 

 tions on its surface. 



These three species illustrate the probable manner in which the 

 ( 'vessels of the test" in Ascidiidse arose phylogenetically ; at first 

 few, short and completely fertile (e.g. Clavelina producta), they sub- 

 sequently increased somewhat in number and extent, dividing 

 dichotomously in the thickness of the test, and became less fertile 

 {e.g. Pycnoclavella aurilucens) ; at a still later stage (represented 

 by Sluiteria rubricollis) the tubes became completely sterile, and, 

 though still not numerous, were essentially organs of the test. The 

 loss of the power of blastogenesis altogether would now bring us to 

 the stage occupied to-day by the species of Ciona ; while an increase 

 in the number of the vessels would lead to the condition found in the 

 greater number of simple Ascidians. 



It is interesting to note also that these forms furnish confirmatory 

 evidence of the view enunciated by Delia Valle f that the sterile ecto- 

 dermic tubes of the test have essentially a " palliogenic " function. 

 In Pycnoclavella aurilucens the past of the test traversed by them is 

 much thicker and firmer than the thoracic portion, and in Sluiteria 

 rubricollis the test is, according to Van Beneden, thicker and more 

 resistant than in Ecteinascidia. The test of " social Ascidians " 

 generally is characteristically thin and soft, and this can be referred 

 directly to the absence or very slight development of sterile 

 " palliogenic " tubes. The softness and delicacy of the test of Ciona 

 as compared with that of Ascidia is also a further confirmation of 

 Delia Valle's view. 



A fully illustrated account of the anatomy of Pycnoclavella will 

 appear in another journal later in the year, and with it will be 

 published coloured sketches of the living colony. 



* E. van Beneden, Les genres Ecteinascidia, Rhopalwa et Sluiteria; note pour 

 servir a la classification des Tuniciers, Bull. Acad. Boy. des Sci., &c , Bruxelles 

 (iii), xiv, 1887, pp. 43, 44. 



f See Arch. Zool. Exp., x, Notes et Revue, p. xli. 



