166 



EMBRYOLOGY 



from an aboral one. The eyes, as well as the fundament 

 of the nervous system lying imder them, are present in the 

 anterior, dorsal part of the body. Behind the middle, ven- 

 tral appendage, the pharynx is already seen (Fig. 77). 

 The intestine also is already established, and appears 

 branched ; in short, the internal organization of the larva 

 corresponds nearly to that with which we became acquainted 

 in the recently hatched embryo of Discocelis. 



The larvfe circle around in the water by the aid of their 

 cilia, revolving upon themselves in various directions. The 

 older, more elongated larvse, on the 

 contrary, are always seen swim- 

 ming with the anterior part of the 

 body directed upwards. They ro- 

 tate around the long axis only. 



After the larva? have swarmed 

 for a considerable time, they ex- 

 change their primitive ovate form 

 for a more and more elongated one. 

 Fig. 77 (probably a larva of Thysa- 

 ')LOzoün) represents a stage older 

 than the embryos just hatched 

 from the egg, which are more com- 

 pact. The elongation of the body 

 is accompanied by a broadening- 

 of the anterior and a narrowing of 

 the posterior end (Fig. 78 A). It can be recognized from 

 Fig. 78 A that, in spite of the presence of the larval appen- 

 dages, the form of the worm is already expressed. This is 

 still more the case in the stage represented by Fig. 78 B, in 

 which the larval appendages are rapidly degenerating. 

 These finally disappear altogether, and the form which 

 characterizes the adult animal is reached by the gradual 

 completion of the internal organization, the increase in the 

 number of eyes, the outgrowth of the nervous system into 

 the longitudinal nerve-trunks, the differentiation of the 

 pharyngeal apparatus and the rest of the muscular system 

 from the mesoderm, and the development of the intestine, 

 with its branches. 



Fig. 77. — Müllek's larva seen 

 from the ventral side (after Joh. 

 Müller, from Balfour's Com- 

 parative EmhnjoJooy). Tlieheavy 

 line indicates the ciliated band. 

 Hi, mouth; u.l, the so-called 

 upper lip. 



