198 THE MICROSCOPE AND ITS REVELATIONS. 



put forth from the segments, and the animal gradually assumes the like- 

 ness of its parent; a few days being passed by the tubicolar kinds, how- 

 ever, in the actively moving condition, before they settle down to the 

 formation of a tube. 1 



596. To carry out any systematic observations on the embryonic 

 development of Annelids, the eggs should be searched for in the situa- 

 tions which these animals haunt; but in places where Annelids abound, 

 free-swimming larvae are often to be obtained at the same time and in the 

 same manner as small Medusae ( 522); and there is probably no part of 

 our coasts, off 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 (Fig. 408) bears a strong resemblance in many particulars 

 to the ' bipinnarian ' larva of a Star-fish ( 543), having an elongated 

 body, with a series of ciliated tentacles (d) symmetrically arranged; 

 these tentacles, however, proceed from a sort of disk which somewhat 

 resembles the 'lophophore 5 of certain Polyzoa ( 549). The mouth (e) 

 is concealed by a broad but pointed hood or ' epistome ' (a), which some- 

 times close down upon the tentacular disk, but is sometimes raised and 

 extended forwards. The nearly cylindrical body terminates abruptly at 

 the other extremity, where the anal orifice of the intestine (b) is sur- 

 rounded 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 itself together, so as to bring the anal extremity 

 and the epistome almost in contact. It is so transparent that the whole 

 of its alimentary canal may be as distinctly seen as that of Laguncula 

 ( 549); 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. This very interest- 

 ing creature was for a long time a puzzle to Zoologists; since, although 

 there could be little doubt of its being a larval form, there was no clue 

 to the nature of the adult produced from it, until this was discovered by 

 Krohn in 1858 to be a Gephyrean W rm - 2 An even more extraordinary 

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

 received the name Pilidiuin (Fig. 409); its shape being that of a helmet, 

 the plume of which is replaced by a single long bristle-like appendage 

 that is in continual motion, its point moving round and round in a 

 circle. This curious organism, first noticed by Miiller, has been since 

 ascertained to be the larva of the well-known Nemertes, a Turbellarian 



or four times its previous length, and the ciliated disk entirely disappeared. An 

 accident unfortunately prevented the more minute examination of this Worm, 

 which the Author would have otherwise made; but he may state that he is cer- 

 tain that there was no fallacy as to the fact above stated; this larva having been 

 placed by itself in a cell, on purpose that it might be carefully studied, and having 

 been only laid aside for a short time whilst other selections were being made 

 from the same gathering of the Tow-net. 



1 For further information on this subject, see Balfour's " Comparative Embry- 

 ology," Chap, xii., and the Memoirs there cited. 



24 Ueber Pilidium und Actinotrocha' in " Mutter's Archiv," 1858, p. 293. 

 For more recent observations upon this interesting creature, see Balfour's ' ' Com- 

 parative Embryology," Vol. i., pp. 299-302, and a paper on 'The Origin and Sig- 

 nificance of the Metamorphosis of Actinotrocha,' by Mr. E. B, Wilson (of 

 Baltimore), in " Quart, Journ, Microsc, Sci." April, 1881. 



