DEVELOPMENT OF ANTEDON. 



153 



nervous system. The embryo usually leaves the egg-membrane 

 on the seventh day, and becomes the free-swimming larva, 

 which shows bilateral symmetry, the ventral surface being 

 slightly flattened. 



The anterior ciliated ring is incomplete ventrally, and between 

 the second and third, which are separated by a wider interval 

 than the others, there is a ciliated depression (Lm), called the 

 vestibular depression (so-called larval mouth) and supposed to 

 correspond with the now closed 

 blastopore. On the ventral surface 

 between the first and second rings, 

 there is a small pit, the adhesive pit 

 (Gr), by the secretion of which the 

 larva, after a free-swimming life of 

 from 12 to 48 hours, attaches itself. 

 After attachment the larva at first 

 lies with its entire ventral surface 

 turned towards the surface to which 

 it is attached ; soon however it erects 

 itself and projects at right angles to 

 the substratum. The attached, i.e. 

 anterior, end of the larva now be- 

 comes narrow and elongated into the 

 stalk, while its free, i.e. posterior, 

 end becomes broader and constitutes 

 the rudiment of the calyx (Fig. 115). 

 The vestibular depression has during 

 these changes become cut off from 

 the ectoderm and forms a closed ecto- 

 dermal vesicle (Fig. 114, 4), which constitutes the larval vestibule. 

 At first this vesicle is placed on the ventral surface, but soon it 

 comes to occupy the free end (Fig. 115). This change in posi- 

 tion is shared by certain internal organs, and is doubtless due 

 to the relative growth of parts by which the ventral surface 

 of the larva comes to occupy the free end. The floor of the 

 vestibule applies itself to the adjacent internal organs and 

 eventually becomes perforated by the mouth opening. Mean- 

 while processes from the water-vascular ring, the tentacular 

 canals, push before them the floor and project as tentacles into 

 the cavity of the vestibule. Eventually the roof of the vesti- 



FlG. 109. Larva of Antedon rosacea 

 with ciliated bands, anterior tuft of 

 cilia and rudiments of the skeletal 

 plates (from Korschelt and 

 Heider). Gr. adhesive pit by 

 which the larva attaches itself; 

 Lm. the vestibular depression 

 (larval mouth). 



