498 Miscellaneous, 



tentacular sac. (In both this sac originates from an invagination 

 of the ectoderm situated at the level of the third circle of cilia, 

 "which afterwards loses its relation with the ventral surface, to open 

 at the apex of the anterior extremity.) But this theory is com- 

 pletely contradicted by another character of higher value, which 

 consists in the situation of the two primitive apertures of the embryo, 

 which I have indicated in a preceding notice. Starting from this 

 fundamental character we arrive at a new conception which it now 

 remains for me to explain. 



Development shows us that the closure of the blastopore is effected 

 not far from the spot where the aperture of the calyx will after- 

 wards appear, and that the ventral pit (fossette ventrule) of authors 

 corresponds in situation with the buccal invagination of the other 

 Echinoderm-larvoe. From this it results that, instead of regarding 

 the region of the calyx as anterior and the region of the peduncle 

 as posterior, wo must, on the contrary, regard as anterior the portion 

 of the larva which becomes the peduncle, and as posterior that por- 

 tion which becomes the calyx, so that the Pentacrinoid cannot be 

 considered to originate from a larva attached by its posterior part, 

 but, on the contrary, from one fixed by its anterior part, by its 

 preoral lobe. 



If we now pass to the homologies, we find that this tj'pe of deve- 

 lopment can only be compared with the larvae of which the entire 

 posterior part becomes transformed into an Echinoderm, while their 

 anterior part is of provisional existence. 



Of this number are the Echinids and Starfishes. Investigations 

 still unpublished upon the metamorphosis of the Sea-Urchins have 

 led me to conclude that the larva (leaving out of consideration the 

 purely accessory organs, the arms, and ciliary fringe) must be 

 regarded as composed of two parts — the anterior formed by the por- 

 tion projecting above the subumbrclla, which includes the preoral 

 lobe plus the oesophageal region ; the posterior composed of all the 

 rest of the body. In the metamorphosis the former of these two 

 parts becomes detached at its base, while the whole of the second 

 becomes transformed into a Sea-Urchin. 



In these two constituent parts of the pluteus of the Echinids we 

 may see portions corresponding to the two fundamental divisions 

 (calyx and peduncle) of the larva) of Comatulte ; their destiny is the 

 same, only the anterior caducous region of the Echinid pluteus 

 never attaches itself and falls earlier. 



As to the concordance above indicated between the evolution of 

 the tentacular sac of the Comatula) and Holothurians, it seems 

 equally to exist between the Comatula) and the Sea-Urchins, in 

 which we find the homologue of this sac in the part described by 

 Metschnikoif under the name of amnios, — Coniptes Rendus, Nov. 8, 

 1886, p. 892. 



