THE TURBELLARIA 



In the case of metamorphic forms, the embryo, however, instead 

 of assuming the flattened character of a Polyclad before leaving 

 the shell, acquires eight processes of the body, arranged in a 

 definite way round the mouth (Lang has compared these lobes and 

 the bands of cilia upon them with the eight rows of swimming plates 



of Ctenophora). They constitute a 

 preoral band, or more correctly, a 

 circumoral band (Fig. XIX. 8, 9). 



This cephalotroch larva, or 

 "Miiller's larva," as it is termed 

 (Fig. XX.), after a free swimming 

 life, is transformed into a young 

 Polyclad by the gradual diminution 

 of the ciliated lobes. 



The larva of Styloclius pilidium, 

 owing to the great development 

 of the dorsal surface, the unequal 

 development of the various ciliated 

 lobes, comes to resemble the typi- 

 cal Nemertine larva, "Pilidium"; 

 and it is possible that this is more 

 nearly like the common ancestor of 

 Turbellaria and Nemertina, while 

 Miiller's larva has gone along a 

 special line in the former group. 

 Balfour showed that the trocho- 

 sphere and other larval forms were 

 readily derivable from these, which 

 can be easily derived from a coelen- 

 terate ; on this account he placed 

 these larvae near to the ancestor of the whole group of Coelomata. 



FIG. XX. 



Miiller's larva of Yiingia, seen from 

 the oral surface. (After Lang, from 

 v. Graft.) 



LITERATURE OF THE TURBELLARIA. 



1. Baer, v. Beit. z. Kemitn. d. nieder. Thier. Nova Acta, xiii. pt. 2, 1827, 



p. 527. 



2. Bergendal. (Uterus of Triclads.) Leuckart's Festschrift, 1892, p. 310. 



3. Ibid. (Polypostiidae.) Rev. Biol. d. Nord. d. 1. France, v. 1893, pp. 237 



and 366. 



4. Bohmig. (Graffilla.) Zeit. f. Wiss. Zool. xliii. 1886, p. 290. 



5. Ibid. (Alloiocoela.) Zeit. f. Wiss. Zool. li. 1891, p. 167. 



6. Ibid. Die Tiirbell. Acoela d. Plankton Exped. 1895. 



7. Ckichkoff. (Freshwater Triclads.) Arch. d. Biol. xii. 1892, p. 435. 



8. Collingwood. (Acanthozoon.) Trans. Linn. Soc. Lond. (2), Zool. i. 1875, 



9. Dalyell. Observ. on some interesting Phenomena in Animal Physiology, 



exhibited by several sp. Planarians, 1814. 

 10. Ibid. The Powers of the Creator, etc., ii. 1853, p. 95. 



