UROCHORDA. 447 



The Urochorda are divisible into the three following Orders : 



I. Laruacea ( = Copelatae). Free-swimming ; body small, more or less oval, 

 provided with a long swimming tail, containing a notochordal rod ; a temporary 

 gelatinous case, secreted by the ectoderm, which is renewed from time to time ; 

 two stigmata : Appendicularidae (Oikopleura, Fritillaria, Kowalewskaid). 



II. Ascidiacea. Either sessile, and then simple, social, or compound, or free- 

 swimming and colonial. Test well developed, and often massive; stigmata 

 numerous, and pharynx large and specialised. 



(1) Sub-order Ascidiae Simplices, fixed or free, solitary or social : sometimes 

 producing by gemmation colonies, in which the Ascidiozooids are connected by a 

 common vascular system, but each retains its own test. The oral and inhalent 

 apertures are near to one another, at the same end of the body. Molgulidae (free 

 as a rule), Cynthiidae, Ascidiadae, and Clavellinidae (colonial). 



(2) Sub-order Ascidiae Compositae. Fixed ; colonial ; Ascidiozooids imbedded 

 in a common test ; often connected by a common vascular system ; and generally 

 grouped more or less regularly in systems round a central cavity, into which the 

 exhalent apertures of the zooids open. Body simple {Botryllidae} ; divisible into a 

 ' thorax ' and ' abdomen ' (Didemnidae) ; or into a ' thorax,' * abdomen,' and ' post- 

 abdomen ' (Polydinidae). Gemmation universal. 



(3) Sub-order Ascidiae Salpaeformes. Free-swimming pelagic colony, in the 

 form of a hollow cylinder, closed at one end and open at the other, at which its 

 extension takes place. Zooids placed perpendicularly to the surface, with the oral 

 aperture external, the cloacal internal, and leading into the central hollow of the 

 cylinder. Pyrosomidae with one genus Pyrosoma, which is phosphorescent. 



III. Thaliacea. Free-swimming ; more or less barrel-shaped, with oral and 

 cloacal apertures at opposite ends of the body ; test very thin ; muscles circularly 

 arranged ; and viscera contracted into a small compass, and laterally placed. An 

 alternation of generations. Doliolidae (Cyclomyaria) the muscles in complete 

 hoops; two transverse rows of stigmata; Doliolum; Anchinia. Salpidae (=.Desmo- 

 myarid) muscular hoops incomplete and sometimes uniting ; pharynx reduced to 

 the dorsal lamina, on either side of which is a large space ; Salpa. 



Larvacea, Fol, Etudes sur les Appendiculaires, Geneva, 1872. Vertebration of 

 tail, Ray Lankester, Q. J. M. xxii. 1882. Ovogenesis, &c., in Appendicularia, 

 A. B. Lee, Recueil Zool. Suisse i, 1884. 



Asddiacea, see pp. 106-7. Ascidians of Provence, Roule, An. Mus. Hist. Nat. 

 Marseille, ii. 1884: Id. Recueil Zool. Suisse, iii. 1886; Id. A. Sc. N. (6) xx. 1886. 

 On place of Clavellinidae, Herdman, Proc. Roy. Soc. Edin. x. p. 716. Development 

 of Clavellina, Seeliger, J. Z. xviii. 1885. Ascidiae Compositae, Herdman, Challenger 

 Reports, xiv. 1886, and Nature, xxix. 1883-84. Synascidiae, Giard, A. Z. Expt. i. 

 1872 ; ii. 1873. Botryllus, Krohn, A. N. 35, 1869. 



Pyrosoma, Keferstein und Ehlers, Zool. Beitrage, Leipzig, 1861 ; Huxley, 

 Tr. L. S. xxiii. 1862 ; Kowalewsky, A. M. A. xi. 1875. 



Thaliacea. Doliolum, Uljanin, Fauna und Flora des Golfes von Neapel, x. 1884. 

 Salpa; testis and alternations of generation, Salensky, Z. W. Z. xxx. Suppl. 1878; 

 gemmation, Id. M. J. iii. 1877; Seeliger, J. Z. xix. 1886; development of ovum, 

 Salensky, Mittheil. Zool. Stat. Naples, iv. 1883. Anchinia. Wagner, A. Z. Expt. (2) iii. 

 1885 ; Korotneff, Z. W. Z. xl. 1884 ; Kowalewsky and Barrois, A. N. H. (5) xii. 1883. 



Egg and envelopes in Urochorda, Fol; Sabatier, Recueil Zool. Suisse, i. 1884; 



