56 REPORT ON THE TUNICATA OF PLYMOUTH. 



and Sluiter), whose transparent test is traversed by several sterile 

 stolonial tubes, branching dichotomously and terminating in a few 

 delicate papillary prolongations on its surface. 



These three species illustrate the probable manner in which the 

 " vessels of the test " in Ascidiidee arose phylogeuetically ; at first 

 few, short and completely fertile (e. g. Clavelina jproducta) , they 

 subsequently 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 ruhricollis) 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 Gio7ia ; 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* that the sterile ecto- 

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

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

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

 ruhricollis 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 ov'i very slight development of sterile 

 " palliogenic ^' tubes. The softness and delicacy of the test of Giona 

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

 Delia Valleys 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. 



Family 2.— PEROPHORID^. 



Body undivided into thorax and abdomen ; viscera on the left 

 side of the branchial sac. 



Test transparent, for the most part thin and membranous, rarely 

 traversed by a few sterile stolonial tubes ; never investing the 

 stolons in a common basal sheath ; apertures generally well apart, 

 the branchial terminal and the cloacal dorsal, lobed, or rarely proxi- 

 mate, terminal and only indistinctly lobed. 



la classification des Tuniciers, Bull. Acad. Roy. des Sci., &e., Bruxelles (iii), xiv, 1887, 

 pp. 43, 44. 



* See Arch. Zool. Exp., x, 1882, Notes et Reinte, p. xli. 



