4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. JZ 



cerned, and when this nervous system is removed from- the surface 

 the removal is efifected by invagination. 



But the only nervous system found in the echinoderms which in 

 its details is at all comparable to the central nervous system of the 

 other higher invertebrates is the so-called apical nervous system of 

 the crinoids, which first forms relatively late in life, and which ap- 

 pears to arise in connection with the coelomic epithelium. 



It seems to me that this nervous system of the crinoids, which is 

 possibly (though not by any means probably) represented by the so- 

 called mesodermal nerve plexus in the starfishes, but which is quite 

 unrepresented in the other echinoderms, afifords the best indication 

 of the probable affinities of these animals, and at the same time its 

 high state of development suggests that the crinoids have departed 

 less widely from the ancestral type than have the other classes. 



In the developing crinoid the ectoderm of the surface of the body 

 becomes more or less disintegrated, and its cells to a greater or lesser 

 extent pass inward and intermingle with the cells of the underlying 

 mesoderm so completely that they can in no way be distinguished 

 from them, the body wall being formed of an ectoderm-mesoderm 

 complex in which the cells of the two types cannot be differentiated. 



On the inside of this body wall, apparently in connection with the 

 coelomic epithelium, the apical nervous system arises ; but in view 

 of the mixed nature of the component cells of this wall it seems not 

 illogical to assume that this apical nervous system is in reality 

 formed from cells which, originally ectodermal, have infiltrated 

 through the underlying mesoderm and now appear as if they belong 

 to the coelomic epithelium. If this hypothesis can be accepted it is 

 obvious that the apical nervous system of the crinoids is in no way 

 comparable to the nervous system of the chordates, and as this is the 

 only echinodermal nervous system comparable with the nervous 

 system of other animals it naturally follows that no affinity with the 

 chordates can be inferred from the nervous system of the other 

 echinoderms. 



In the crinoids the larvae become attached by the ventral side of the 

 anterior end, and the column is a development of the preoral lobe, aris- 

 ing therefore from the place where in the larvae the anterior nerve 

 mass, just in front of the mouth, is situated. The mouth moves from 

 the ventral surface onto the left side and then migrates upward, 

 away from the point of attachment, until it comes to lie at the pole 

 opposite the latter, that is, at the posterior end near the anus. 



