PRAEORAL LOBE. 2/1 



The very interesting observation has recently been 

 made by MACBRIDE, that the larva of Astcrina gibbosa 

 actually undergoes temporary fixation at the beginning of 

 the metamorphosis, the fixation being effected by the 

 praeoral lobe in a manner strikingly similar to that of the 

 larvas of Ante don and of dona. 



In the larva of Antedon the adhering disc, by which the 

 larva eventually fixes itself to some foreign surface, is 

 placed near the front end of 

 the prseoral lobe immediately 

 below the apical plate. 



The central nervous sys- 

 tem of the adult Echinoderm 

 arises in entire indepen- 

 dence of the actual or sup- 

 pressed apical nervous sys- 



Fig. 130. Larva of Asterina gibbosa, 



tem Of the larva, and not at viewed as an opaque object from the left 

 ,, f ., j r . i side. (After LUDWIG.) 



all from the ectoderm of the j r j PnEoral lobe . 

 praeoral lobe. 



We have thus seen how within the limits of a single 

 group (viz. the Echinoderms) the praeoral lobe can become 

 completely emancipated from the central nervous system ; 

 and we have further recognised the fact that whether the 

 cavity of the praeoral lobe is a derivative of the primary or 

 secondary body-cavity, whether it contains loose mesen- 

 chyme or is lined by an endothelium, the morphological 

 value of the praeoral lobe itself remains the same. 



THE PR^ORAL LOBE OF THE PROTOCHORDATES. 



It is probable that the misunderstandings and disagree- 

 ments which are of such frequent occurrence among mor- 

 phologists with regard to the comparison of the types of 

 central nervous system presented respectively by the 



