DEVELOPMENT 459 



ganglion, and in some species four eye-spots ; at the base of this, 

 but behind the mouth, is a ring of larval tentacles. The prae- 

 oral lobe and the tentacles are ciliated ; the margin of the lobe 

 bears, however, specially long cilia, and there is also a ring of 

 long cilia around the anus. 



Before the Actinotrocha stage has been reached the larva 

 has forsaken the shelter of its parent's tentacles, and swims 

 actively about in the open sea. As it grows older a finger-like 

 involution of the skin (c) arises just behind the tentacles on the 

 ventral surface and grows into the body, increasing greatly in 

 length and becoming much folded. The larva now sinks to the 

 bottom of the sea, and after swimming round many times on its 

 axis, undergoes a very astonishing metamorphosis (Fig. 231, B, 

 C). The finger -like involution is suddenly turned inside out, 

 and forms a large projection on the ventral surface, into which 

 the alimentary canal passes, assuming a U -shape, as in the adult. 

 This ventral process in fact forms all the body of the adult behind 

 the line of tentacles, and subsequently contains, not only the 

 alimentary canal, but the kidneys, the reproductive organs, and a 

 large part of the vascular system. At the same time the prae- 

 oral lobe breaks off, and, together with its ganglia and eye-spots, 

 passes into the mouth and is digested in the stomach ; the larval 

 tentacles follow the prae-oral lobe, and are similarly digested. 

 Their place is taken by a ring of adult tentacles which commence 

 to appear just behind the larval tentacles before they fall off. 

 The animal is now practically adult. 1 



It is obvious that this astonishing metamorphosis is accom- 

 panied by the rotation of the axes of the animal. The adult 

 practically lives at right angles to the larva. In the latter the 

 anus marked the posterior end, and the prae-oral lobe the anterior. 

 The prae-oral lobe has disappeared in the adult, but its position 

 is marked by the mouth. The ventral surface has enormously 

 increased, and corresponds with the whole surface of the trunk. 

 To be consistent we must therefore regard the mouth of the 



1 Caldwell, loc. cit. Foettinger, Arch. Biol. vol. iii. 1882, p. 679 ; Gegenbaur, 

 Zeitschr. iviss. Zool. vol. v. 1854, p. 345 ; Krohn, Arch. Anat. Jahrgang 1858, 

 p. 289 ; Metschnikoff, Nachricht. k. Ges. Wiss. Gbttingcn, No. 12, 1869, p. 227, 

 and Zeitschr. wiss. Zool. vol. xxi. 1871, p. 233 ; J. Midler, Arch. Anat. Jahrgang 

 1846, p. 101 ; Schneider, Monatsber. Ak. wiss. Berlin, 1861, p. 934, and Arch. 

 Anat. Jahrgang 1862, p. 47 ; Wagener, Arch. Anat. Jahrgang 1847, p. 202 ; Wilson, 

 Amer. Natural, vol. xiv. 1880. 



