ENTEROPNEDSTA 383 



ciliated band, both uf whicli are provided with several 

 flexures (Fig. 169 ^-1). They almost come in contact with 

 each other at the anterior end of the larva. At this point 

 is found an ectodermal thickening, comparable to the apical 

 plate of the Annelid larvae, with two eye-spots lying over it. 

 From this region a contractile band extends backwards. 

 Mesenchymatous cells seem to arise between the intestine 

 and the body- wall. The posterior part of the larva is en- 

 circled by a ring of cilia which is independent of the other 

 ciliated band (Fig. 169 A), and in later stages of the larva 

 another one may also make its appearance behind this. 



[The ciliated band of the anterior part of the body may undergo more 

 or less extensive outfoldings, by which the external form of the larva is 

 considerably modified. These outgrowths are sometimes large, such as 

 we shall find in the larvas of Echinoderms, and these influence, as has 

 been said, the entire form of the body. Continual outgrowths of the 

 ciliated band of limited extent result in the formation of tentacle-like 

 structures. — K.] 



The condition of the larva described is only gradually 

 reached during its free pelagic life. At first the transverse 

 (posterior) rings of cilia are lacking, and the preoral and 

 post-oral ciliated bands have a more simple course. In the 

 further development of the Tornaria its anterior end elon- 

 gates and becomes the acorn of the Balanoglossus. Preoral 

 and post-oral ciliated bands then disappear, and instead of 

 these the entire body becomes covered with cilia (Fig. 

 169 B). The eye-spots are still retained for a while at the 

 tip of the anterior end. The middle region of the body is 

 encircled by the transverse ciliated band, and it can thus be 

 seen that the parts lying behind it have also elongated. 

 Two openings make their appearance externally on the 

 dorsal side of the anterior part of the body, the external 

 openings of the gill-pockets. With this nearly the same 

 stage is reached which we saw arising by direct means from 

 the larva described by Bateson. The simpler mode of de- 

 velopment is doubtless to be considered as the derived, and 

 that of the Tornaria as the more primitive, since the absence 

 of mouth and anus in a free-swimming larva does not 

 represent a primitive condition. 



