128 DEVELOPMENT. 



meres (cells), which begins immediately after fertilization and is 

 speedily completed (Morula stage). The 3^olk assumes the 

 appearance of a cluster of round nucleated cells, the large ones 

 internal, the smaller external ; forming the embryo — except a 

 portion which remains as nutritive material. The j^olk first 

 loses its spherical form, elongates somewhat, develops upon its 

 entire surface a delicate ciliary covering, and begins its wonderful 

 rotary movement.* At the anterior part of the body a circlet or 

 crown of long .cilia arises ; the portion of the body supporting 

 this is then elevated into a ridge, then a ring, and finally it 

 develops on each side into a rounded lobe. Both lobes together 

 present somewhat the appearance of the figure 8 : these are the 

 vela which Forskal first described as the organs of locomotion of 

 the univalve molluscan larvae. Most of the prosobranchiate and 

 ppisthobranchiate larvEe are velamentous. Immediately below 

 the velum the mouth is developed as an invagination ; at the 

 posterior end the anus is similar formed ; both open into the 

 intestinal cavity which has been formed by the displacement of 

 the large yolk-cells in the centre. An intestine is now present, 

 and the large cells which are somewhat heaped up posteriorly, 

 become in large part the liver and intestinal wall. A body- 

 cavity between this large-celled intestinal wall and the small- 

 celled body-wall is not j^et present, and originates later by a 

 separation of the two walls and the appearance of a fluid 

 between them. 



Below the mouth the foot arises as a blunt ciliated appendage, 

 whilst the ciliated covering of the rest of the body has become 

 lost. The velum, that at first surrounded the fore-part of the 

 body is more elevated to the dorsal side after the appearance of 

 the mouth, since the mouth is not placed in the middle of the 

 velum, but beneath its narrow portion, and is dorsally overhung 

 by the velum, whilst ventrally the foot extends beyond it. The 

 body becomes more elongate, and soon at its posterior dorsal 

 portion the delicate, hyaline, cup-like shell appears, in which also 

 a distinct laminated structure may be detected ; at the posterior 

 part of the foot, the operculum appears at the same time. The 

 sense organs are now developed ; at each side in the velum the 

 tentacle arises as a papilla ; internally by the side of the 

 oesophagus the auditory vesicles arise, the otoliths then appear 

 therein ; and immediately thereafter, or at the same time, along- 

 side of the tentacles the e^^es are formed, which, at first, lik#the 

 organs of hearing are vesicles, lined with cilia internally, the 

 lens appearing later. The central portions of the nervous sj^stem, 

 the ganglia around the oesophagus, now become visible. Along 



* The blastoderm becomes invaginated, and we see the gastrula stage in 

 the opisthobranchiate genera Polycera, Doris, etc. 



