Comparison of Echinoderm Larvæ and General Conclusions.
In any comparison of the various types of Echinoderm larvæ it is necessary to distinguish between the free-swimming forms, and the viviparous or fixed forms. A very superficial examination suffices to shew that the free-swimming forms agree very much more closely amongst themselves than the viviparous forms. We are therefore justified in concluding that in the viviparous forms the development is abbreviated and modified.
All the free forms are nearly alike in their earliest stage after the formation of the archenteron. The surface between the anus and the future mouth becomes flattened, and (except in Antedon, Cucumaria, Psolinus, etc. which practically have an abbreviated development like that of the viviparous forms) a ridge of cilia becomes established in front of the mouth, and a second ridge between the mouth and the anus. This larval form, which is shewn in [fig. 264] A, is the type from which the various forms of Echinoderm larvæ start.
In all cases, except in Bipinnaria, the two ciliated ridges soon become united, and constitute a single longitudinal post-oral ciliated ring.
The larvæ in their further growth undergo various changes, and in the later stages they may be divided into two groups:
(1) The Pluteus larva of Echinoids and Ophiuroids.
(2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) type.
The first group is characterized by the growth of a number of arms more or less surrounding the mouth, and supported by calcareous rods. The ciliated band retains its primitive condition as a simple longitudinal band throughout larval life. There is a very small præ-oral lobe, while an anal lobe is very largely developed.
Fig. 271. A. The larva of a Holothuroid. B. The larva of an Asterias.