430 



NATURE 



{March 6, 1884 



tion which has been in alcohol for about ten years. 

 Laborious dissection and the preparation of large numbers 

 of sections are necessary to reveal characteristics which 

 may often be seen in the living specimen by observation 

 merely. And, what is of more consequence, there is a 

 risk of being led into errors and misinterpretations by 

 the abnormal contraction and distortions caused by the 

 alcohol. 



Such plates as those of Prof. Giard,' and of Dr. R. von 

 Drasche's beautifully illustrated monograph on the Synas- 

 cidiffi of the Bay of Rovigno,'- which has just appeared, show 

 how much can be made out from a natural representation 

 of the living animal, and leave little or nothing to be desired 

 so long as we must be content with some substitute for the 

 actual specimen. In this important worlc von Drasche 

 criticises Giard's classification of the Synascidis, and ex- 

 plains fully a scheme of his own which appeared in the 

 Zflologisclier Anseigcr for 1882. Many attempts have 

 been made to classify nattirally this difficult group, and 

 this latest effort, although it has corrected some previous 

 errors, appears still to be susceptible of improvement, 

 especially as regards the interesting forms which occupy 

 the borderland between simple and compound Ascidians. 

 Some of these (the Clavelinidae) are placed by Giard and 

 von Drasche in the Synascidia;, while in 18S0 I tried to 

 show that their proper position was amongst the AscidiK 

 Simplices, and close to the genus Ciona. At the present 

 moment I confess that I am unable to find a single satis- 

 factory character by which to distinguish these two hrge 

 groups, the simple and compound Ascidians. 



Savigny, in 1815, in his " Observations sur les Alcyons 

 gi^latineux a six tentacles simples,"^ first rescued the 

 compound Ascidians from the Alcyonaria with which they 

 had previously been associated, and demonstrated their 

 affinity with the other Tunicata'' In the "Tableau Systd- 

 matique " Savigny gives no formal statement of the cha- 

 racters distinguishing the two groups, but it is evident 

 from some passages in his "3' Mdmoire" that he relied 

 chiefly, if not entirely, for their separation upon the 

 arrangement of the Ascidiozooids of the compound forms 

 around a central cloaca — a character which he declared 

 WIS visible even in the young embryo. In this latter 

 point he was mistaken, and it seeins rather singular that 

 he should have laid such stress upon the union of the 

 atrial apertures when we find that he describes and figures 

 their separate and independent existence in DhiMiui and 

 Distoma, two of the genera of his " Tifthyes Composdes." 

 Llavelina in his system is placed next to the " Phallusia: 

 Cionae" (=the modern genus Ciona) in the Ascidias 

 Simplices. 



Savigny classified the nine genera which he recognised 

 amongst compound Ascidians by means of characters 

 taken from the branchial and atrial apertures. But 

 although such characters are most useful and constant 

 marks of affinity in the simple Ascidians, they fail signally 

 as applied by Savigny to the compound forms, and result 

 in the separation of his closely allied genera Didemniiiii 

 and Euaclii/ni, while Diazona, Distoma, and Sigilliini 

 are thrown together in one group, and Eucaelium is 

 placed with Botryiius, a genus with which it has certainly 

 no close relationship. 



Lamarck's arrangement of the Tunicata, published 

 about the same time, showed no improvement upon that 

 of Savignv. 



In 1841 Milne-Edwards^ estabhshed the group of 

 " Ascidise Sociales'' as occupying an independent position 

 between the simple and compound forms. This group 



I " Recherches sur les Ascidies Compos^es ou Synascidies " (Archive!: de 

 i^oologie ex/'himcntate ct Generate, t i 1872). 



- •• Die --vnascidien der Bucht von Ravigno." Ein Beitrag jur Fauna der 

 Adria, von Dr. Richard von Drasche (Wien. iSS^I. 



3 '■ M^moires sur les Anim. sans Vert.' , „ . 



' The class Tunicata was established by Lamarck in the year folUwmg— 



5 •' Observations sur les Ascidies Composees des Cotes de la Man:hc " 

 (.1/A«. Instit. France, vol. xv.ii.). 



(in wirich he placed the genera Pcroplwra and Chivelitia) 

 he defines as comprising Ascidians which reproduce by 

 buds as well as by eggs, and which live united by common 

 radiciform prolongations, but which otherwise are free of 

 all adhesion to one another. He distinguished the simple 

 Ascidians as forms which never reproduced by gemma- 

 tion and were never found in groups united by a common 

 tegumentary tissue ; while he separated the compound 

 from the social Ascidians on account of their possessing 

 a test common to all the members of the colony. If we 

 unite the simple and social .Ascidians, which I have 

 shown in the Report upon the Clialtciig;r Tunicata there 

 is reason for doing, we shall have, according to Milne- 

 Edwards, the simple and compound Ascidians dis- 

 tinguished merely by the members of the colony in the 

 latter being united by a common test, while in the former 

 each individual has its own distinct tunic. This character, 

 although better than the one made use of by Savigny, is, 

 as we shall see later on, by no means an infallible guide. 



Milne-Edwards formed a classification of the genera ot 

 compound Ascidians into " Polycliniens," '■ Didemniens," 

 and " Botrylliens," which, with our present knowledge of 

 the group, still seems fairly natural. These three divisions 

 are distinguished by such anatoaiical characters as the 

 relations of the other viscera to the branchial sac. In the 

 " Polycliniens" the body has three regions — the "thorax," 

 containing the branchial sac; the "abdomen," formed 

 by the stomach and the greater part of the intestine ; and 

 the " post-abdomen," having the reproductive organs and 

 the heart. In the "Didemniens" there are only two 

 regions — thorax and abdomen— the reproductive organs 

 and heart being placed on the intestine. In the third 

 group, the " Botrylliens," the viscera form a single mass, 

 in which the alimentary canal lies alongside the branchial 

 sac. 



This arrangement of the Ascidiaj Composita: was 

 generally accepted until 1872, when Giard published ' his 

 important memoir, " Recherches sur les Ascidies Com- 

 passes ou Synascidies," in which is given a classification 

 based upon the method of gemmation. He distinguishes 

 three points of origin for the buds — the pyloric region of 

 the alimentary canal, the reproductive organs, and the 

 posterior end of the body. The latter region is the place 

 of gemmation in his " Catenate," a group which contains 

 three families — the Clavelinida: the Perophoridae, and 

 the Botryllidre. But he gives no sufficient reasons for 

 placing the first two families in the compound Ascidians, 

 and, as von Drasche has pointed out, the third one does 

 not really exhibit the essential character of the CatenatEe. 

 Giard's second group, the " Glomeratae," is charac- 

 terised mainly by the formation of ovarian buds. It cor- 

 responds to Milne-Edwards' " Polycliniens," in addition 

 to half of the " Didemniens." The remainder of the 

 " Didemniens " correspond to Giard's third group, the 

 " Reticulata;," and are characterised by gemmation 

 taking place from the pyloric region. This seems a 

 natural and well-defined section, including two families, 

 the Didemnida: and the Diplosomidas, but the " Glom- 

 eratae" cannot stand without several chinges which von 

 Drasche suggests, and which really reduce it merely to 

 Milne-Edwards' section " Polycliniens." Upon the whole, 

 there can be little doubt that Milne-Edwards' classifica- 

 tion is preferable to that proposed by Giard. 



We come now to Dr. von Drasche, the latest authority, 

 who, both in his preliminary note" and in the detaileil 

 memoir,-' wisely abstains from any attempt to form main 

 divisions, and merely groups the genera in a series of 

 carefully chosen families. Of these the Botryllidffi corre- 

 sponds to Milne-Edwards' section "Botrylliens," while 

 the Didemnidas and Diplosomida: are identical with 

 Giard's families bearing the same names. The Polyclinidaa 



I Arch de Zool. expir., t. i. 



- Zoologhcher AuzcUer{ox 1882. p. 605. 



'i "Die Synascidien der Bucht vjn Rovigno" (Wien, iSSs)- 



