158 



TURBELLARIA. 



The peripheral ciliated layer of small cells becomes divided into 

 two strata, of which the outer remains ciliated and forms the true epi- 

 blast : the inner probably forms the cutis. In it are developed rod-like 

 bodies, which seem to be homologous with the thread cells of the 

 Ccelenterata, so that if the views put forward in the previous chapter as 

 to the similarity of the turbellarian and coelenterate larvse are correct, 

 the cutis corresponds with the deeper layer of the coelenterate epiblast. 

 The mesoblast, like the epiblast, becomes divided into two strata. 

 The outer one is stated to form the circular and longitudinal muscles ; 

 the inner one to give rise to a muscular reticulum, the spaces 

 within which constitute the parenchymatous body cavity. 



The later changes are not of great importance. At a period slightly 

 after the formation of the mouth and ganglia two pairs of stiff hairs become 

 formed at the sides of the body. The embryo has by this time grown 

 so as to fill up its capsule, in which however it continues rapidly to 

 rotate, and also commences to exhibit active contractions. It next 

 becomes hatched, and passes from a spherical to a flattened elongated 

 form. The ventral oral opening is at first central, but soon, by a process of 

 unequal growth, becomes carried towards the posterior end of the body. 

 The pairs of stiff hairs in the meantime considerably increase in number. 

 The remains of the yolk cells now disappear, and the enteric walls become 

 more distinct. The alimentary canal, which is at first simple in outline 

 like that of a rhabdocoelous Turbellarian, soon assumes a dendritic form. 

 The young animal after these changes resembles its parent, except in the 

 possession of only two pairs of eyes and in the absence of generative organs. 



Of the types with a complete metamorphosis the free Iarva3 of 

 various species of Thysauozoon have been observed by Job. Miiller 



(190) and Moseley (189), and the com- 

 plete development of Eurylepta auri- 

 culata has been studied by Hallez. 



The stages within the egg of this 

 latter type agree precisely with those 

 already described in Leptoplana. After 

 the formation of the mouth the body 

 elongates, remaining however cylin- 

 drical. A fold forms on the anterior 

 side of the mouth, giving rise to a 

 large upper lip. Two posterior pro- 

 cesses are next formed, and other 

 processes soon arise, constituting the 

 whole of those found in the free larva. 

 The embryo next shakes off its egg 

 membranes by a series of vigorous con- 

 tractions. When free it has the form 

 represented in the annexed figure (fig. 

 87). 



It is so similar to Muller's (fig. 88) and Moseley's larvae that all 

 three may be dealt with together. 



FIG. 87. LARVA OF EURY- 

 LEPTA AURICULATA IMMEDIATELY 

 AFTER HATCHING. VlEWED FROM 



THE SIDE. (After Hallez.) 

 m. mouth. 



