346 AETHUR WILLEY. 



fact that it has undoubtedly been specially elaborated in the 

 Ascidians to serve as a larval locomotor organ. In Amphi- 

 oxus, although there is a post-anal portion of the body, there 

 is no tail sensu stricto, locomotion being effected by 

 serpentine movements of the whole body. 



In view, therefore, of the above considerations I regard the 

 tail of the Ascidian tadpole as homologous with the tail of 

 Vertebrates generally, and the conclusions to which Oscar 

 Hertwig has recently come (19) as to the nature of the Verte- 

 brate tail must be taken as applying equally well to the tail of 

 the Ascidian tadpole. 



Hertwig says (p. 457), *' Wir finden in seiner Zusammen- 

 setzung nur dorsal gelegene Organe betheiligt, Nervenrohr, 

 Chorda, Ursegmente, wahrend Leibeshohle, Geschlechts- 

 organe, Nieren, sich nicht in ihn hinein fortsetzen. Ob wir 

 von einem wirklichen Schwauzdarm reden dilrfen, erscheint 

 mir fraglich. Allerdings verlangert sich das innere Keimblatt 

 als ein Strang in die Schwanzanlage hinein, als eine Ausstiil- 

 pung der Beckeudarmwand. Es scheiut aber meist nicht zur 

 Ausbilduug einer Hohlung zu kommen und spater schwindet 

 der Strang und lost sich in andere Gewebe auf. Bei keinem 

 Wirbelthier wird er wohl als Darm functionirt 

 haben, so dass ich vorschlage, den Namen Schwauzdarm ganz 

 fallen zu lassen und ihn durch die zu keiiien falschen Vorstel- 

 lungen flihreude Bezeichnung 'Entodermstrang des Schwanzes' 

 zu ersetzen.'' 



How then, considering the different topographical relations 

 in the two cases, can the Ascidian intestine be homologous 

 with that of Amphioxus? In answer to this question I 

 can only apply the principle which Lang (25) applied in his 

 discussion of the relations between Cephalodiscus and Balano- 

 glossus, and say that the Ascidians are forms allied to Amphi- 

 oxus which have adapted themselves to a sessile habit of life 

 (festsitzende Lebcusweisc), this adaptation leading, as in so 

 many other cases, to a U-shaped alimentary canal. In raauy 

 cases, e. g. Pyrosoma, &c., the sessile habit has been secondarily 

 exchanged for a pelagic one. 



