362 Dr. 0. Hamann on the Phylogeny 



of the Annelida from Medusse, speculations and ideas which 

 can hardly find confirmation in nature. The larval forms of 

 the Echinodermata, the formation of the bodj-cavitj, the 

 enterocoele, the origin and structure of the nervous system, 

 will chiefly point towards worm -like creatures, and indeed to 

 such forms as possess a typical enterocoele of like origin and 

 development^ and in which the nervous system is either still 

 situated in the ectoderm, as in the Asterida, or arranged in 

 the same way as in the Echinida and Holothurige. To ascribe 

 to the Echinodermata a near relationship to the Coelenterata, 

 as has been done by Kleinenberg, although certainly only in 

 a remark en passant *, will not do, for the agreement in the 

 structure of the nervous system alone would not suffice to 

 balance the great number of other diiferences of structure, as, 

 for example, the existence of a body-cavity in the Echino- 

 dermata. 



Among the groups of Vermes the Annelida, with their 

 typical enterocoele, stand nearest to the Echinodermata, as 

 Hackel has long since shown ; and, in fact, this notion is 

 most particularly well supported, especially by the structure 

 of the body-wall. In the Asterida there is in each arm a 

 dermal muscular tube, consisting of a layer of annular and a 

 layer of longitudinal muscles. In the Echinida the former 

 exists only in a rudimentary form (Ludwig), while in the 

 Ilolothuriaj it appears to be confined to definite zones. 



As regards the structure of the nervous system, it is as 

 simple as is conceivable in Asterida, consisting of epithelial 

 sense-cells and nerve-fibres. But among the Vermes also, 

 and, indeed, among the more highly developed of them, we 

 find forms in which the whole nervous system persists through- 

 out life in the ectoderm. This is the case in the Archanne- 

 lida (Hatschek and Fraipont). 



There is consequently no reason to prevent us from regard- 

 ing the Echinodermata, although not as Annelida, yet as 

 descending from Vermes provided with true body-cavities, in 

 which the nervous system still remained in the lowest stage 

 of development and in which a water-vascular system was 

 probably already developed. But then the first question is. 

 What group of Echinodermata is to be regarded as the 

 earliest, and are the different divisions deducible from each 

 other ? 



It is remarkable that the majority of zoologists and geolo- 

 gists regard the Crinoidea (or Oystidea) as those which have 

 retained all organizational characters in their most primitive 

 condition. 



* Zeitschr. fiir wis?. Zool. Bd. xliv. 



