570 ECHINODERM-LARV,E \ BIPINNARIA J PLUTEUS. 



Pluteus of the Echinida (§ 442). Tlie temporary Mouth of the 

 Larva does not remain as the permanent mouth of the Star-fish : 

 for the (Esophagus of the latter enters on what is to become the 

 dorsal side of its body, and the true mouth is subsequently formed 

 by the thinning-away of the integument on its ventral surface. 

 The young Star-fish is separated from the Bipinnarian Larva by 

 the forcible contractions of the connecting stalk, as soon as the 

 Calcareous consolidation of its integument has taken-place and it3 

 true Mouth has been formed, but long before it has attained the 

 adult condition ; and as its ulterior development has not hitherto 

 been observed in any instance, it is not yet known what are the 

 species in which this mode of evolution prevails. The Larval 

 Zooid continues active for several days after its detachment; and 

 it is possible, though perhaps scarcely probable, that it may de- 

 velope another Asteroid by a repetition of this process of gem- 

 mation. * 



442. In the Bipinnaria, as in other Larval Zooids of the 

 Asteriada, there is no internal Calcareous frame-work ; such a 

 frame-work, however, is found in the larva? of the Eehiuida and 

 Ophiurida, of which the form delineated in Fig. 297 is an 

 example. + The Embryo issues from the Ovum as soon as it has 

 attained, by repeated ' segmentation ' of the Yolk (§ 476), the 

 condition of the ' mulberry-mass ; ' and the superficial cells of this 

 are covered with Cilia, by whose agency it swims freely through the 

 water. So rapid are the early processes of development, that no 

 more than from twelve to twenty-four hours intervene between 

 Fecundation and the emersion of the Embryo ; the division into 

 two, four, or even eight segments taking-place within three hours 

 after impregnation. Within a few hours after its emersion, the 

 Embryo changes from the spherical into a sub-pyramidal form 

 with a flattened base ; and in the centre of this base is a depres- 

 sion, which gradually deepens, so as to form a Mouth that com- 



* See the observations of Koren and Daniellsen (of Bergen) in the " Zoo- 

 logiske Ridrag," Bergen, 1847 (translated in the "Ann. des Sci. Nat.," 

 tier. 3, Zool., Tom. iii., p. 347 : and the Memoir of Prof. Miiller, 'Ueber 

 die Larven und die Metamorphose der Echinodermen,' in "Abhaldlungen 

 dei- Koniglichen Akademie der Wissenschaften zu Berlin," 1848. — Another 

 very dissimilar mode of development in certain Star-fish was first de- 

 scribed by Sars in his " Fauna littoralis Norvegise," 1846, and has been 

 since investigated by Busch (" Beobachtungen fiber Anatomie und 

 Entwickelung einiger Wirbellosen Seethiere," 1851), Prof. Miiller ("Uber 

 den allgemeinen Plan in der Entwickelung der Echinodermen," 1853), 

 and Prof. Wyville Thomson (' On the Embryology of Aster acantkion 

 violaceus') in "Quart. Joum. of Microsc. Science," N.S., Vol. i. (1861), p. 99. 



t See Prof. Miiller, ' Ueber die Larven und die Metamorphose der 

 Ophiuren und Seeigel,' in " Abhaldlungen der Koniglichen Akademie der 

 Wissenschaften zu Berlin," 1846. See also, for the earlier stages, a Memoir 

 by M. Derbes, in "Ann. des Sci. Nat.," Ser. 3, Zool., Tom. viii., p. 80; 

 and for the later, Krohn's "Beitrag zur Entwickelungsgeschichte der 

 Seeigillarven," Heidelberg, 1849, and his Memoir in "Mailer's Arcbiv.," 

 18J1. 



