3846 E. W. MACBRIDE. 
the posterior wall and ventral edge of the larvalorgan. This is 
well shown in the sagittal section, fig. 81. The larva increases 
in size, and the prezoral portion and larval organ alter their 
shape, the latter changing from a circular to an elongated 
elliptical form, whilst the przoral lobe extends in a vertical 
direction (Pl. 18, figs. 7 to 9). The whole larva has now the 
form which Ludwig calls slipper-shaped, but which would be 
more correctly termed boot-shaped, the dorsal lobe of the preoral 
lobe representing the toe and the ventral one the heel of the 
boot. In the centre of the larval organ appears an elevation 
(fiz.). This structure, which Ludwig did not interpret, we shall 
find to have a most important function during the metamor- 
phosis; it is, in fact, the disc by means of which the animal 
fixes itself. Possibly this disc also functions during free life 
for temporary attachment, though in a different manner ; thus 
when the larval organ is applied to the substratum, the retrac- 
tion of this disc would cause a cupping action which would be 
relieved by its again being protruded. It has been pointed out 
by Ludwig, and I have myself confirmed it again and again, 
that the larva is able to attach itself most strongly to the sub- 
stratum. The mode of life of the larva Ludwig calls ‘‘creeping.” 
This is not strictly correct; as far as I have seen, the larva 
swims by means of the cilia of the larval organ. The latter is 
directed downwards, and for this reason Ludwig calls what I 
have termed the anterior surface of the animal the ventral, and 
the posterior end becomes for him the dorsal end. I cannot 
agree with this orientation ; the proper longitudinal axis of any 
bilaterally symmetrical animal is the oro-anal one, and it is by 
this that I discriminate between the dorsal and ventral, the 
anterior and posterior surfaces. That the posterior end is held 
upwards is no more reason for calling it dorsal than the fact 
that the Cephalopod directs the apex of its visceral hump back- 
wards is reason for calling that posterior. I should mention 
that Ludwig calls the whole przoral portion of the body, the 
preoral lobe in fact, the larval organ. I wish to avoid this, 
since the preoral lobe has functions which Ludwig did not 
suspect, and hence I confine the term “ larval organ” to the 
