353 



LARVAL FORMS. 



changes the most important are (i) the early appearance 

 round the anal end of the body of a ring of bristles ; and (2) the 

 appearance of a pair of ventral setse in the anterior part of the 

 body. The anterior ring of bristles characteristic of the adult 

 Echiurus does not appear till a late period. 



Of the internal changes the earliest is the formation of the 

 anal respiratory sacks. With the growth of the posterior part 

 of the trunk the intestine elongates, and becomes coiled. 



Bonellia. The embryo of Bonellia, while still within the 

 egg, retains a spherical form and acquires an equatorial band of 

 cilia, behind which a second narrower band is soon established, 

 while in front of the first one a pair of eye-spots becomes 



aL.. 



FIG. 162. THREE STAGES IN THE DEVELOPMENT OK BONELLIA. (After Spengel.) 



A. Larva with two ciliated bands and two eye-spots. 



B. Ripe larva from the dorsal surface. 



C. Young female Bonellia from the side. 



al. alimentary tract ; m. mouth ; sc. provisional excretory tube ; s. ventral hook ; 

 an.i'. anal vesicle. 



formed (fig. 162 A). The embryo on becoming hatched rapidly 

 elongates, while at the same time it becomes dorso-ventrally 

 flattened and acquires a complete coating of cilia (fig. 162 B). 

 According to Spengel it resembles at this time in its form and 

 habits a rhabdocoelous Turbellarian. The anterior part is 

 however somewhat swollen and presents an indication of a 

 prse-oral lobe. 



