CHAPTER XIV. 



ECHINODEKMATA.i 



Development in the five divisions of the Echinodermata 

 offers so much in common that we shall treat of these 

 together as far as possible. In the development of the 

 Echinodermata we distinguish the following four periods : — 



1. The formation of the primary germ-layers and the 

 mesenchymal together with the establishment of mouth and 

 anus. 



2. The origin of the enterocoele and hydrocoele. 



3. The formation of the typical larval form. 



4. The metamorphosis of the larva into the Echinoderm. 



I. THE FORMATION OF THE PRIMARY GERM-LAYERS 

 AND THE MESENCHYMA, TOGETHER WITH THE 

 ESTABLISHMENT OF MOUTH AND ANUS. 



As far as is known, the cleavage of the Echinoderm egg 

 is always total. In the Holothurioidea (Synapta), more- 

 over, it is strictly equal ; whereas in the star-fishes and 

 sea-urchins it takes place less regularly. Within the mass 

 of cells which has arisen by cleavage there is found even 

 during this period a cavity which, in ,the further course of 

 cleavage, continues to enlarge, and becomes an extensive 

 blastocoele. The result of cleavage is always a ccbIo- 

 hlastula. The next stage of development, too, exhibits 

 an essential agreement in the different groups of Echino- 

 derms, for in all of them it consists of an invagination 

 gastrula. In the details, however, certain deviations from 

 the common plan of development occur in the different 

 forms. 



^ The maturation and fertilization of the Echinoderm egg will be 

 discussed in the general part. 



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