67 
pole and indeed the Ascidians would not be the only sessile 
animals which attach themselves to their substratum with 
the animal pole. The same can be said of the sessile Coe- 
lenterata, Cirripedia and Echinoderm-larvae, perhaps of more 
forms also. At the fixation of the Ascidian larva the praeoral 
nevropore lobe swells up enormously and 
then presents the aspect of a large 
cavity bounded by ectoderm and 
containing loose, scattered, meso- 
derm cells. The fixing papillae 
flatten out and give place to a 
thickened disc of ectoderm at the 
anterior end of the praeoral lobe. 
Mouth of Ascidians. — If our 
conception of the praeoral lobe 
and the neuropore in Ascíidia is 
correct, the mouth of the Ascidian 
larvae can be homologous neither 
to that of Amphioxus nor to that 
of Craniates, for it opens at a 
place corresponding exactly to the 
neuropore, the old mouth. Accord- 
ing to HUNTSMAN (1913) a con- 
siderable part of the epithelium 
of the wall of the oral cavity is 
derived directly from the primitive 
neural tube of the embryo (fig. 
27), so that in fact the new mouth 
in these larvae is nothing but the 
old mouth, though a shorter com- 
munication with the archenteron 
Fig. 27. Diagrammatic has been established. 1 hardly 
sagittal sections through need emphasize once more, that 
se bhp nk PE Or As the circumstance that in the three 
Gikatortaatten: ot dd, main groups of Chordates three 
tube into wall o: cral different mouths are foun 
siphon (after HUNTSMAN, strongly pleads for the supposi- 
deld tion that the mouth of Chordates 
is a secondary formation, a consequence of the loss of 
the primary mouth, of which process my theory gives such 
A simple explanation. 
Segmentation in Ascidians. — If now we try to account 
for the further structure of the Ascidian larva and the 
