134 PHYLUM ECHINODEBMATA. 



egg is small and the entire development takes place in the sea, 

 and in most of them there is a free-swimming, externally bisym- 

 metrical larval stage. In about fifty species however the young 

 remain in connexion with the mother during their development 

 and care of the brood occurs, and in some cases (e.g. Asterina 

 gibbosa) a free-swimming larva is not formed. In such cases the 

 egg is usually larger than when a free-swimming larva is formed, 

 owing to the presence of a greater quantity of food-yolk.* 

 When care of the brood occurs, | the young may either creep 

 about freely on the surface of the mother, or they may be 

 attached to special parts of the body wall, e.g. the neighbourhood 

 of the mouth, among the spines of the back, in special depres- 



FIG. 87. Two stages in the development of Holothuria tubulosa viewed in optical section 

 (from Balfour, after Selenka). A, Blastosphere at the close of segmentation. B, com- 

 mencing gastrula stage, ae archenteron ; bl wall of blastosphere ; ep ectoderm ; ft vitelline 

 membrane ; hy endoderm ; mr micropyle ; ms protoplasmic immigration into the seg- 

 mentation cavity from the invaginating endoderm ; sc segmentation cavity. 



sions of the skin. In some Holothurians the development takes 

 place in the body cavity, and in the dioecious Chiridota contorta 

 in the genital tubes. In the starfish Stichaster nutrix the young 



339. Id., The development of Echinus esculentus, Phil. Trans., 195, 

 1903, p. 285. H. Bury, The Metamorphosis of Echinoderms, Q.J.M.S., 

 38, 1895. H. Theel, The development of Echinocyamus pusillus, Nova 

 Acta E. Soc. Sci. Upsala, 1892. Id. Prelim. Account of the development 

 of Echinus miliaris, Proc. Eoy. Swedish Acad., 28, 1902. G. W. Field, 

 The larva of Asterias vulgaris, Q.J.M.S., 34, 1893, p. 105. S. Goto, The 

 metamorphosis of Asterias pallida, etc., Journal Coll. Sci. Imp. Univ., 

 Tokyo, 10, 1898, p. 239. Id., Metamorphosis of Asterina gibbosa, Ibid. f 

 12, 1898, p. 227. Th. Mortensen, Echinodermlarven, Ergeb. d. Plarikton- 

 Exped. d. Humboldt-Stiftung, 2, 1898. 



* In Benthodytes sanguinolenta the eggs measure 2 mm. in diameter. 



f Ludwig, Zoolog. Jahrb., Suppl. Bd., 7, 1904, p. 683. 



