GEPHYREA, 



297 



traced. It is continuous with the supra-oesophageal band which com- 

 pletely girths the oesophagus without exhibiting any special dorsal eularge- 



B 



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



A. Larva with two ciliated bauds arid two eye-spots. 



B. Eipe 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.v. anal vesicle. 



meiit. After the ventral cord has become completely separated from the 

 epiblast a central fibrous mass becomes differentiated in it, while the lateral 

 parts are composed of ganglion cells. In the arrangement of its cells it 

 presents indications of being composed of two lateral halves. It is, 

 however, without ganglionic swellings. 



The mesoblast, though at first very thin, soon exhibits a differentiation 

 into a splanchnic and somatic layer though the two do not become 

 distinctly separated by a body cavity. The somatic layer rapidly becomes 

 thicker, and enlarges laterally to form two bands united dorsally and 

 venti*ally by narrow, thinner bands. The outermost parts of each of these 

 bands become differentiated into an external circular and an internal 

 longitudinal layer of muscles. In the prse-oral lobe the mesoblast assumes 

 a somewhat vacuolated character. 



The hypoblast cells form a complete layer round the four yolk cells 

 from which they arise (fig. 162 B, al), but at first no alimentary lumen is 

 developed. The oesophagus appears during this period as an, at first 

 solid, but subsequently hollow, outgrowth of the hypoblast towards the 

 epiblast. 



The metamorphosis of the larva into the adult female Bonellia 

 commences with the conversion of many of the indifferent mesoblast 

 cells into blood corpuscles, and the introduction into the body cavity 

 of a large amount of fluid, which separates the splanchnic and 

 somatic layers of mesoblast. The fluid is believed by Spengel to be 

 sea-water, introduced by two anal pouches, the development of which 

 is described below. 



