CH^ETOPODA. 331 



Larvae do not necessarily continue to belong to the same group 

 at all ages. A larva may commence as a monotrochal form and 

 then become telotrochal and from this pass into a polytrochal 

 condition, etc. 



The atrochal forms are to be regarded as larvae which never 

 pass beyond the primitive stage of uniform ciliation, which in 

 other instances may precede that of definite rings. They usually 

 lose their cilia early, as in the cases of Serpula and other larvae 

 described below. 



The atrochal larvae are not common. The following history of an 

 Eunicidan larva (probably Lumbriconereis) from Claparede and Metschni- 

 koff (No. 336) will illustrate their general history. 



In the earliest stage noticed the larva has a spherical form, the prae-oral 

 lobe not being very well marked. In the interior is a globular digestive 

 tract. The cilia form a broad central band leaving free a narrow space at 

 the apex of the pne-oral lobe, and also a circumanal space. At the apex of 

 the prse-oral lobe is placed a bunch of long cilia, and a patch of cilia also 

 marks out the anal area. 



As the larva grows older it becomes elongated, and the anterior bunch of 

 cilia is absorbed. The alimentary canal divides itself into pharynx and 

 intestine. The former opens (?) by the mouth in the middle of the central 

 band of cilia, the latter in the anal patch. The setae indicating the segmen- 

 tation are formed successively in the posterior ring-like area free from cilia. 

 The cilia disappear after the formation of two segments. 



In Lumbricus, the embryo of which ought perhaps to be grouped with 

 the atrochae, the cilia (Kleinenberg) cover a ventral tract of epiblast between 

 the two mesoblastic cords, and are continued anteriorly to form a circle 

 round the mouth. 



The monotrochal larvae are provided only with the important 

 prse-oral ciliated ring before mentioned. In the majority of 

 cases they are transitional forms destined very shortly to become 

 telotrochal, and in such instances they usually have a more or 

 less spherical body which is nearly divided into two equal halves 

 by a ciliated ring. In some few instances, such as Polynoe, 

 Dasychone, etc., the monotrochal characters are not lost till the 

 larval cilia are exuviated. 



The telotrochal forms (of which examples are shewn in figs. 

 144, 150, etc.) may (i) start as monotrochal; or (2) from the 

 first have a telotrochal character ; or (3) be derived from atrochal 

 forms. The last mode of origin probably represents the ances- 

 tral one. 



