HABITS, ETC., OF CEJiEBRATaLlJS LAOTEUS. 155 



invagination becomes the oral surface, while that which is 

 for the present the superior portion of the embryo is the 

 aboral surface. These correspond to the terms "sub- 

 umbrella'^ and " umbrella/^ as used by Salensky (45). The 

 opening in the centre of the oral surface — the blastopore — is 

 the only opening into the invaginated entoderm. It is per- 

 manent, and functions as both mouth and anus during the 

 life of the larva, and is probably transformed into the mouth 

 of the adult as in other species, a new anus being formed. 

 The entire inner surface of the entoderm is covered with cilia 

 as soon as they appear on the ectoderm. The cilia around the 

 edge of the oral surface are much larger and longer than 

 elsewhere on the pilidium, and form what is known as the 

 pre-oral ring, which corresponds with those on trochophore 

 and turbellarian larvae. 



At this period the larva consists of a single outer layer of 

 cubical ectoderm and an inner layer of invaginated columnar 

 entoderm, also single. The space between these — the body- 

 cavity — is filled with a colourless, transparent, gelatinous 

 liquid. 



Invagination is thus embolic, and the first stage in the 

 formation of the pilidium is accomplished. 



The Apical Plate. — As soon as invagination becomes 

 distinct the embryo falls over on its side. At the same time 

 a slight depression appears near the centre of the aboral sur- 

 face (fig. 8), which is saucer-shaped at first, but becomes 

 markedly cup-shaped in later development. It never lies 

 exactly at the centre, but always a little anterior to it on the 

 median line. 



For this reason it marks the first appearance of bilateral 

 symmetry in the embryo, which has hitherto preserved a dis- 

 tinctly radial form (cf. Korscheldt and Heider). 



In the centre of the depression there is developed what at 

 first sight appears to be, and what was long mistaken for, a 

 single very strong flagellum. It is really a bunch or tuft of 

 flagella, twenty to thirty in number, which are nearly always 

 held firmly together. The depression, the apical plate, is 



