VIII 



EGHINODERMA TA ONTOGENY 



523 



Transformation of the Pluteus larva into the young Echinoid. This meta- 

 morphosis is far from having been satisfactorily described, its investigation being 

 exceedingly difficult. 



An important part in the shaping of the Echinoid body is played by the above- 

 mentioned flask-like invagination of the ectoderm on the left side. The thicken- 

 ing floor of this invagination grows towards the hydroca'l, and becomes externally 

 applied to this latter as the "Echinoid disc" (Fig. 420). The thin lateral walls of 

 the capacious flask, which is still connected by its neck with the larval ectoderm, 

 are known as the amnion. 



The hydroccel vesicle assumes the horse -shoe -shape, and at the same time puts 

 forth five outgrowths which push before them the Echinoid disc, i.e. the floor of 



ant 



ii 



Fio. 422. Lateral view of a very 

 young Echinoid (Echinocyamus 

 pusillus), forty -live days old (after 

 Theel). The first tube-feet and spines 

 of the Echinoid are seen, and, attached 

 to its back, the remains of the calcar- 

 eous rod of a larval arm. 



Fio. 4-21. Dorsal aspect of a larva of Echi- 

 nocyamus pusillus, about forty-five days old 

 (after The'el). 1, The larval anus, with their 

 calcareous rods ; 2, unpaired calcareous rod, 

 taking part in surrounding the dorsal pore. (3) ; 

 4, spines ; 5 and fi, primary tentacles of the 

 young Echinoid ; a tit, anterior ; post, posterior ; 

 sin, left ; dex, right. 



the flask-like invagination. Five hollow tubes thus now project into the cavity of 

 the flask, which continually becomes more and more spacious ; these are the five 

 primary tentacles, which receive their covering from the spreading Echinoid disc. 

 This Echinoid disc forms the oral wall (that is, no doubt, only the epithelium and 

 the nerves ?) of the young Echinoid, while the apical wall is formed direct from the 

 larval dorsal ectoderm of the Pluteus. 



The fate of the amnion is differently described for different forms. Sometimes 

 it is said to pass over into the young Echinoid, the amnion sac opening and spread- 

 ing out, and yielding the circular integumental region between the apical and oral 

 surfaces of the body. At other times, again, the amnion sac is said to remain closed 



