310 



EMBRYOLOGY 



of the body, which thus approaches the form of the adult 

 worm, as can be seen from the larval stage of Thalassema 

 shown in Fig. 147. Even in this stage the ciliated bands are 

 still present. With the gradual reduction of these, which 

 soon takes place, the larva approaches 



/^ ^ more and more the shape of the adult 



^ animal. The oral and preoral parts of 



^ the larva are transformed into the pro- 



stomium, whereas the hind part still 

 grows in length. 



^ 



3, Larval Form and Metamor- 

 phosis of Bonellia. 



The larva of Bonellia resembles the 

 Trochophore less than do the larvae of 

 Echiurus and Thalassema, although it 

 also can be referred to the Trochophore. 

 Evidently it is much modified, as is 

 proved by its internal organization, 

 which is less adapted to a free life. 

 We follow Spemgel in describing the 

 development of Bonellia. 



The larva of Bonellia, which is at first 

 spherical, possesses, in addition to the anterior 

 band of cilia, a posterior one (Fig. 148^). 

 m^^m^,^ The anterior to all appearances corresponds to 



^^^K. - - flj the anterior ciliated band of the larva of 



\ Thalassema and Echiurus, especially since in 



the latter a band of cilia is also found in the 

 region of the anus. Anteriorly two eye-spots 

 make their appearance (Fig. 148 A and h). 

 The intestine is not yet differentiated as such, 

 but is developed later. The larva increases in 

 length, becomes flattened dorso-ventrally and 

 covered with cilia (Fig. 148 B). It now has more the appearance of a 

 Turbellarian, and, as its shape indicates, it moves by creeping. 



The further development of the larva affects first its internal organi- 

 zation. The supra-cesophageal ganglion has become differentiated from 

 the ectoderm, and later the oesophageal connectives and the ventral 

 nerve cord begin to develop. The entoderm has become a single layer 



P! 



Fig, 147.— Late larval 

 stage of Thalassema mdlita 

 (after Conn), a, anus ; ah, 

 anal vesicle ; d, intestine ; 

 m, mouth ; vh, ventral er 

 uncinate setae. 



