236 THE HOLOTHUR1OIDEA 



Little is known concerning the origin of the class. It may, 

 however, safely be conjectured that the Holothurians are derived 

 from an Echinoderm ancestor with five typically developed radii, 

 along which ran branches of the nervous and water-vascular systems, 

 the latter provided with podia. The outward bilateral symmetry 

 of the Holothurians seems to have been secondarily acquired in 

 connection with their free-moving mode of life. 1 



LIST OF WOIIKS. 



1. Baur, A. 1864. (Beitrage zur Naturgeschichte der Synapta digitata.} 



Nova Acta Acad. Leop. -Carol, vol. xxxi. 



2. Bury, If. 1889. (Studies in the Embryology of Echinoderms.) Quart. 



Journ. Micr. Sci., n.s., vol. xxix. pp. 409-449, pis. xxxvii.-xxxix. 1895. 

 (The Metamorphosis of Echinoderms.) Op. cit. vol. xxxviii. pp. 45-135, 

 pis. iii.-ix. 



3. Hamann, 0. 1883. (Beitrage zur Histologie der Echinodermen. I. Die 



Holotlmrien (Pedata), u.s.w.) Zeit. f. Wiss. Zool. vol. xxxix. pp. 

 145-190, pis. x.-xii. (II. Das Nervensystem der Pedaten Holotlmrien. 

 Die Cuvierschen Organe. Nervensystem und Sinnesorgane der Ape- 

 daten), torn. cit. pp. 309-333, pis. xx.-xxii. 



4. Htrouard, E. 1889. (Recherches snr les Holothuries des cotes de France.) 



Arch. Zool. Exper. (2), vol. vii. pp. 535-704, pis. xxv.-xxxii. 



5. Ludwig, H. 1889-92. (Die Seewalzen.) Bronn's Klassen und Ordmmgen 



des Thierreichs, Echinodermen, I. Klasse. [Gives a full Bibliography.] 



6. 1894. (Reports on an exploration ... by the . . . Albatross . . . 



XII. The Holothurioidea. ) Mem. Mus. Harvard, vol. xvii. No. 3. 



7. Sclenka, E. 1867. (Beitrage zur Anatomic und Systematik der Holo- 



tlmrien.) Zeit. f. Wiss. Zool. vol. xvii. pp. 291-374, 4 pis. 



8. 1876. (Zur Entwickelung der Holotlmrien.) Ibid. vol. xxvii. pp. 155- 



178, pis. ix.-xiii. 



9. Scmon 9 R. 1888. (Die Entwickelung der Synapta digitata.) Jena. Zeitschr. 



vol. xxii. pp. 175-309, pis. vi.-xii. 



10. Semper, C. 1868. Reisen in Archipel der Philippines II. Theil. Vol. i. 



Holothurien, Leipzig. 



11. Theel, Hj. 1882, 1886. (Report on the Holothurioidea.) Challenger 



Reports, Zoology, vol. iv. part xiii., and vol. xiv. part xxxix. 



12. Ticdcmann, F. 1816. Anatomie der Rb'hren-Holothurie, u.s.w. Fol. 



Landslmt. 



See also Nos. 2, 3, 19, 25a, and 28, in Literature of Echinoderma generally, 

 Chapter VIII. p. 35. 



1 See Chapter VIII. It should not be forgotten that the plane of symmetry of 

 the adult does not correspond exactly \vith that of the larva, and that the ali- 

 mentary canal and the mesentery are invariably twisted spirally from left to right. 



