228 B. W. MACBRIDB. 



the posterior wall and ventral edge of the larval organ. This is 

 well shown in the sagittal section, fig. 31. The larva increases 

 in size, and the praeoral portion and larval organ alter their 

 shape, the latter changing from a circular to an elongated 

 elliptical form, whilst the prseoral lobe extends in a vertical 

 direction (PI. XI, 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 prseoral 

 lobe representing the toe and the ventral one the heel of the 

 boot. In the centre of the larval organ appears an elevation 

 (fix.) . 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 difi'erent 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 prseoral portion of the body, the 

 prseoral lobe in fact, the larval organ. I wish to avoid this, 

 since the prseoral lobe has functions which Ludwig did not 

 suspect, and hence I confine the term " larval organ " to the 



