l34 The ancestry of Vertebrates 



Appendicularians, the first point that striices us is the total 

 loss of segmentation in the "trunk." The question 

 as to whether the regular arrangement of the muscle-cell§ 

 in the tail is to be considered as a last remnant of former 

 segmentation or not, has been a point of controversy, 

 especially recenty between MARTINI (1909) and iHLE (1910). 

 As to the latter question, the facts do not give convincing 

 evidence for either alternative. From the point of view of 

 my theory it appears to be not improbable that traces of 

 former segmentation might be recognizable in the tail, 

 since no doubt we must assume a segmented ancestor. In 

 the trunk, however, no traces of segmentation of the 

 mesoderm are to be noted anymore, nor is there a question 

 of a secondary body cavity or a coelome. 



The only starting point for a further comparison with 

 the segmented Chordates is afforded by tl;e first, and in 

 Appendicularia the only, pair of gill-slits. These still might 

 indicate the limit of two former segments. Are they, if this 

 be the case, situated at the limit of the praeoral lobe and 

 the first segment, as is the mouth of Craniates and the two 

 anterior entoderm diverticula of Amphioxus? In this case 

 they ought to be found directly under the neuro^^ore or the 

 definitive mouth. Alternatively, are they situated at the 

 limit of the first and the second segment? In the latter 

 alternative the question is answered by VAN Wyhe 

 (1893, p. 155): the first pair of gill-slits of Ascidian larvae 

 is situated behind the brain vesicle, as is the mouth and 

 the club-shaped gland in Amphioxus. To both of these, there- 

 fore, they correspond and this conclusion is confirmed by 

 the circumstance that the endostyle both in Amphioxus and 

 in Ascidian larvae originates in front of them, while the 

 rudiment of the thyroid gland in Craniates, which has been 

 compared by MtJLLER (1873, p. 327) and DOHRN (1885, 

 p. 49) to the endostyle, is situated between the mouth and 

 the first pair of gill-slits (VAN WYHE, 1902, p. 141), just 

 as in Amphioxus under the mandibular segment which I 

 consider as the first segment of the soma Thus our con- 

 clusion can only be, thatthefirst pair of gill-slits in Ascidian 

 larvae, the only pair in Appendicularia, indicates the limit 

 of the first and the second body segment. While, according 

 to Van -Beneden and lULIN (1884, p. 360), the Ascidians, 

 in the same way as the Appendicularia, have only one pair of 

 primary gill-slits, WlLLEY (1893, p. 336) derives the numerous 



