10 EMBRYOLOGY OF THE STARFISH. 



of which they are composed. Our young Echinodei'm in this condition 

 (PI. I. Figs. 23-28) can be strictly homologized with the earlier stages 

 of a Polyp at the time when the digestive cavity is first formed, before 

 the appearance of the partitions ; and with an acalephian embryo, where 

 the digestive cavity alone is developed, previous to the pushing of the 

 chymiferous tubes through the gelatinous mass. The stages subsequent 

 to the condition of the embryo here described, represented in PI. I. Fig. 

 24, not having been traced very carefully by previous observers, we have 

 not had before us the means of forming a true conception of the mode 

 of development of the Echinoderms ; for to obtain a clear and precise 

 idea of the functions of those problematic bodies which have puzzled 

 Mviller during the whole of his investigations, it is necessary to follow, 

 step by step, the changes taking place in the pouch of the embryo, which 

 is in this early stage its digestive cavity {d) ; for it is as much a diges- 

 tive cavity as that of a young Actinia or a Scyphistoma, where the 

 same opening serves as mouth and anus. The mode of formation of the 

 digestive cavity is entirely diflerent in the two classes; in the Polyp 

 it is hollowed out of the interior of the embryo, while in the Echino- 

 derm the bending in of the wall forms the stomach. Hence the two 

 cavities are not homologous, and the openings which lead into them, 

 though performing similar functions — those of mouth and anus — are 

 likewise in no way homologous, though they are in all built upon the 

 plan of radiation. This opening always retains its double function in 

 the Polyps and some of the Acalephs, while in the Echinoderms it becomes 

 the anus after the true mouth has been formed, and the currents have 

 ceased to cii'culate in the extremity of the pouch and to pass out through 

 the same opening which admitted them. 



If there is any doubt that Echinoderms, Acalephs, and Polj'ps belong 

 to the same great type of the animal kingdom, a comparison of the 

 young Echinoderm, Acaleph, or Polyp in their earlier stages of growth, 

 at a time when the spherosome has not yet been divided into its com- 

 ponent spheromeres, will show how great is their identity of development, 

 and how little there is in nature to justify the separation of this most 

 natural great division of the animal kingdom, the Radiates, into Echino- 

 derms and Coelenterata. I shall I'eturn to this point when speaking of the 

 homologies of the larvfe of Echinoderms. 



Formation of the Mouth. — The perfect symmetry of the larva (PI. I. 



