332 ARTHUR WILLEY. 



hervor In fig. 40 fanden wir die Endostylwand auf 



dem vorderen Chordaende nahezu senkrecht stehen und 

 mussten sie als vordere bezeichnen. Dieses Verhaltniss finden . 

 wir noch auf dem in fig. 42 gezeichneten Stadium ; von jetzt ab 



aber beginnt die Drehung Als die Ursache dieser 



Verschiebung kann ich nur die Umbildung des vor dem En- 

 dostyl liegenden Ektodermlappens durch eine Einschniirung 

 zum Haftstolo erkennen^ mittelst dessen die Larve sich spater 

 festheftet." Seeliger does not, however, perceive any contra- 

 diction between his description of the primary position of 

 the endostyle in Clavelina with that of Kowalevsky for Phal- 

 lusia mammillata. 



3, The Prseoral Lobe and Endostyle of Ascidians 

 compared with the corresponding Structures in 

 Amphioxus. 



The cavity of the praeoral lobe, or, as it is sometimes called, 

 the snout of Amphioxus, is derived, as is well known from the 

 description given by Hatschek (15), from the right of a pair of 

 anterior intestinal diverticula. 



These diverticula are at first perfectly symmetrical, but soon 

 the right one begins to enlarge, and ultimately gives rise to the 

 large coelomic cavity which occupies the anterior extremity of 

 the larva of Amphioxus (see Lankester and Willey, 28, pi. 

 xxix, figs. 2 and 5, and pi. xxx, fig. 1). 



The left diverticulum remains comparatively small, and then 

 acquires a wide opening to the exterior and becomes the prse- 

 oral pit (Wimperorgan, Hatschek). 



It is necessary to emphasise the fact, which would appear to 

 be sufficiently obvious, that these two anterior intestinal 

 diverticula of Amphioxus are not the first pair of metameric 

 coelomic pouches. They first arise, as shown by Hatschek, at 

 a stage when already some eight pairs of coelomic pouches are 

 present. Their origin, therefore, and subsequent fate show 

 that they are structures sui generis in front of the mouth and 

 in front of the first pair of trunk somites. 



The anterior body-cavity of Amphioxus has, therefore, 



