68 
Appendicularians, the first point that strikes us is the total 
loss of segmentation in the “trunk” The question 
as to whether the regular arrangement of the muscle-cells 
in the tail is to be considered as a last remnant of former 
segmentation or not, has been a point of controversy, 
especially recent:y 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 secondarry body cavity or a coelome. 
The only starting point for a further comparison with 
the segmented Chordates is afforded by the 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 neuropore or the 
deiinitive 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 MüLLER (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 | 
consider as the first segment of the soma Thus our com 
clusion can only be, that the first 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 JULIN (1884, p. 360), the Ascidians, 
in the same way as the Appendicularia, have only one pair of 
primary gill-slits, WILLEY (1893, p. 336) derives the numerots 
