1>LA TYELMINTHES. 1 (>:} 



This is known as the type of Desor ami is confined (1} to the 

 genus Lineus. The Pilidium and the Desor type may be first con- 

 sidered (vide Barrois, No. 192). 



The type of Desor. The segmentation is regular and leads to 

 the formation of a blastosphere with a large segmentation cavity. 

 The blastosphere is converted by invagination into a gastrula (fig. 

 91 A). The blastopore is soon carried relatively forwards by the 

 elongation backwards of the archenteron, and, according to Barrois, 

 actually forms the mouth. Owing to the elongation of the arch- 

 enteric cavity the embryo assumes a bilateral form (fig. 92 A; in 

 which the dorsal and ventral surfaces can be distinguished, the mouth 

 (m.) being situated on the ventral surface. 



Immediately after the completion of the gastrula a remarkable 

 series of phenomena takes place. The embryo when viewed from the 

 ventral surface assumes a pentagonal form (fig. 91 B), and four 

 invaginations of the epiblast make their appearance on the ventral 

 surface (fig. 92 A), two in front of (pr. d.) and two behind (po. d.) 

 the mouth; they result in the formation of four thickened discs. 

 These discs soon become separated from the external skin, which 

 closes in forming an unbroken layer over them (fig. 91 C). Tin- 

 discs grow rapidly, and first the prostomial pair and subsequently the 



FIG. 92. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois. ) 

 A. Side view of an embryo at a very early stage as an opaque object. 

 B and C. Two late stages, seen as transparent objects from the ventral surface. 

 ae. archenteron ; m. mouth; pr.d. prostomial disc; po.d. metastomial disc; cs. lateral 

 pit developing in B as a divt rticnlum from the oesophagus ; pr. proboscis ; ms. muscular 

 layer (?); Is. larval skin about to be thrown off; me. mesoblast; .s-f. stomach. 



11 2 



