950 



WOKMS 



moving condition, before they settle down to the formation of a 

 tube. 1 



To carry out any systematic observations on the embryonic 

 development of Annulata the eggs should be searched for in the 

 situations which these animals haunt ; but in places where Amiu- 

 lata abound free-swimming larvae are often to be obtained at the 

 same time and in the same manner as small Medusae ; and there is 

 probably no part of our coasts oft' which some very curious forms 

 may not be met with. The following may be specially mentioned 

 as departing widely from the ordinary type, and as in themselves 

 extremely beautiful objects : The Actinotrocha, which is now known 



to be the young stage of the Gephyrean 

 worm Phoronis (fig. 716), bears a 

 strong resemblance in many particulars 

 to the ' bipinnarian ' larva of a star- 

 fish, having an elongated body, with 

 a series of ciliated tentacles (d) sym- 

 metrically arranged ; these tentacles, 

 however, proceed from a sort of disc 

 which somewhat resembles the ' lopho- 

 phore ' of certain Polyzoa. The mouth 

 (e) is concealed by a broad but pointed 

 hood or ' epistome ' (a), which some- 

 times closes down upon the tentacular 

 disc, but is sometimes raised and ex- 

 tended forwards. The nearly cylin- 

 drical body terminates abruptly at the 

 other extremity, where the anal orifice 

 of the intestine (b) is surrounded by a 

 circlet of very large cilia. This animal 

 swims with great activity, sometimes 

 by the tentacular cilia, sometimes by 

 the anal circlet, sometimes by both 

 combined ; and besides its movement 

 of progression it frequently doubles 

 FIG. HQ.-Actinotrocha branchi- itself together, so as to bring the anal 

 ata : a. epistome or hood; o, .*? , , , . , , . , 



anus ; c, stomach ; d ciliated extremity and the epistome almost into 

 tentacles ; e, mouth. contact. It js so transparent that the 



whole of its alimentary canal may be 



as distinctly seen as that of Laguncula ; and, as in that polyzoon, 

 the alimentary masses often to be seen within the stomach (c) are 

 kept in a continual whirling movement by the agency of cilia, with 

 which its walls are clothed. 2 An even more extraordinary departure 

 from the ordinary type is presented by the larva which has received 

 the name Pilidium (fig. 717), its shape being that of a helmet, the 



1 For further information on this subject see Balfour's Comparative Embryology, 

 vol. i. chap. xii. and the memoirs there cited. 



2 ' Ueber Pilidium und Actinotrocha ' in Midler's Archiv, 1858, p. 293. For 

 more recent observations upon the latter creature, see Balfour's Comparative 

 Embnjology, vol. i. pp. 299-302 ; and a paper 011 ' The Origin and Significance of the 

 Metamorphosis of Actinotrocha, 1 by Mr. E. B. Wilson (of Baltimore), in Quart. 

 Journ. Microsc. Sci. April 1881. 



