218 



II. Morphology; Phylogeny; Biology. 



The general homology of the four main types of the Echinoderm larvae 

 was so clearly pointed out by J oh. Miiller in his classical memoir "Ober 

 den allgemeinen Plan in der Entwicklung der Echinodermen"i) that very 

 little has had to be corrected 2) or added, the main thing being that the 

 separate frontal band of the Asteroid-larvae does not represent an essential 

 difTerence from the other larvae, as Joh. Miiller thought it to do. In the 

 light of the more extensive knowledge of the larval forms now acquired 

 it may be worth while to pay some attention to the special development 

 within each of the four types, to see how they develop from the primitive 

 generalized shape into more or less highly specialized forms or, on the other 

 hand, are reduced into forms so simple that they are hardly recognizable 

 as forms derived from the true pelagic larval type. 



The Auricularia is the type in which the least amount of specialization 

 appears to have taken place, due allowance being made for the fact that 

 we know comparatively less about the Holothurioid larvae than of the 

 three other main groups. The simplest form is that found within the genus 

 Holothuria ( — unless it should turn out to be much more specialized in 

 the more advanced stages of development, which does, however, not seem 

 very likely — ). The vibratile band hardly shows an indication of the typic- 

 al larval processes, the larva in fact looking almost like a diagram of the 

 primitive Echinoderm-larva, as regards its outer shape ( — as to its inner 

 structure it is less simple — ). Only simple calcareous bodies are known 

 to occur, irregular stars or spherical bodies^), no wheels. — The larvae of 

 Synapta and Stichopus are only a little more speciahzed, the vibratile band 

 forming small processes corresponding to those of the other Echinoderm- 

 larvae. In the Synapta-larvsL calcareous bodies are formed in the shape 

 of elegant wheels. 



The Auricularia paradoxa being in all probability only a decalcified 

 Echinopluteus transversus, only one group of Holothurian larvje is known, 

 which represents a more speciahzed form of the Auricularian type, viz. 

 the larvae Auricularia minor, plicata and nudibranchiata. While A. minor 

 is still comparatively easily referred to the primitive type, A. plicata is 

 considerably more specialized, but the climax is reached by A. nudibran- 

 chiata in which the vibratile band develops to such an extraordinary degree 

 of comphcation that it is difficult enough to recognize the original type. 

 Auricularia antarctica, though somewhat speciahzed in other directions, 



1) Abhandl. d. Akad. BerUn. 1853. Taf. II. 



^) Conip. the author's Memoir "Die Echinodermenlarven d. Plankton-Expedition" p 8 

 ») The Auricularia sphaerigera (Joh. Miillers's "Auricularia mit Kugeln") may well 

 be suggested to belong to the genus Holothuria. 



