[218] The origin of the vaso-peritoneal vesicle is not quite the same in all the species. In Holothuria tubulosa it is separated from the cæcal end of the archenteron; the remainder of which then grows towards the oral invagination. In Cucumaria the archenteron forks ([fig. 249]); and one fork forms the vaso-peritoneal vesicle, and the other the major part of the mesenteron.
[219] There appears to be some uncertainty as to how much of the larval œsophagus is derived from the stomodæal invagination.
[220] Vide P. H. Carpenter, “On the genus Actinometra.” Linnean Trans., 2nd Series, Zoology, Vol. II., Part I., 1879.
[221] The exact position of the madreporic tubercle in relation to the abactinal plates does not seem to have been made out. It might have been anticipated that it would be placed in one of the primary interradial plates, but this does not seem to be the case. The position of the anus is also obscure.
[222] The following statements are taken from the abstract in Bronn’s Thierreichs.
[223] Whether interradial plates are developed as in Asterias is not clear. They seem to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other forms (vide Carpenter, No. [548], p. 369).
[224] Vide especially Müller, Agassiz, and Metschnikoff.
[225] For viviparous Echini vide Agassiz, Proc. Amer. Acad. 1876.
[226] Götte (No. [549]) supported by Müller’s and Krohn’s older, and in some points extremely erroneous observations, has enunciated the view that the radial canals in Echinoids and Holothuroids have a different nature from those in Asteroids and Ophiuroids.
[227] Götte (No. [549]) on the other hand holds that the centro-dorsal plate is developed by the coalescence of a series of at first independent rods, which originate simultaneously with, and close to, the lower edges of the basals, and that it is therefore similar in its origin to the basals.