ECHINODERMATA 457 



The blastopore, in those cases where it persists, becomes 

 the anus. The mouth arises bj the union of the archenteron 

 with the ectoderm. Certain differences occur in regard to 

 the formation of mouth and anus as the result of differences 

 in the mode of life and the consequent alteration of the 

 shape of the larva. The larvae are quite different in shape 

 in the different groups of Echinoderms, although common 

 features are not lacking. Apart from the internal organiza- 

 tion already mentioned, in regard to which they are entirely 

 comparable with one another, the external characters can 

 also be compared, in spite of the differences in the shape of 

 the body and, above all, the ciliated band, which, together 

 with the shape of the larva, is derivable from a common 

 fundamental larval form. Even the cask-shaped larva of 

 Antedon, differing as it does from the other larval forms of 

 the Echinoderms, resembles in shape the so-called pupa of 

 the Holothurians, which, like it, possesses five ciliated bands. 

 The further development must in turn present differences 

 corresponding to the variations in shape of the larvae ; but, 

 on the other hand, it also exhibits certain resemblances, as 

 the usual similarity of structure in the dift'erent systems of 

 organs demands. Thus not only the fundament, but also 

 the further development, of the water-vascular system re- 

 curs in nearly the same way in all cases. The development 

 of the nervous system, as far as it is known, also shows 

 many things in common, and the same is true to a still 

 greater extent for the musculature. The early development 

 and union of the ambulacral and subambulacral surfaces of 

 the starfish and the Ophiuran in the Bmchiolaria and 

 Pluteus larvse, which differ so much from each other in 

 shape, takes place in a strikingly similar way. In the 

 larvae of Crinoids and Holothurians, a certain resemblance 

 iin regard to the development of the tentacles at the bottom 

 of the vestibule or oral funnel cannot be mistaken. 



As regards the development of the skeleton, it is not 



[possible from what is at present known to discover any posi- 



jtively established relations between the different divisions 



of Echinoderms. To be sure, reference has been made to the 



position of the forming plates in relation to the internal 



