274 LARVAL FORMS. 



segment between the two characteristic telotrochal rings. When 



o O 



these rings are complete the larvae are polytrochse proper, when 

 they are only half rings they are either nototrochoo or gasterotrocha?. 

 Sometimes there are both dorsal and ventral half rings which do not 



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however correspond, such forms constitute the amphitrochse. 



In the mesotrochge one or two rings are present in the middle of 

 the body, and the characteristic telotrochal rings are absent. 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 telo- 

 trochal and from this pass into a polytrochal condition, etc. 



The atrochal forms are to be regarded as larvas which never pass 

 beyond the primitive stage of uniform ciliation, which in other in- 

 stances may precede that of definite rings. They usually lose their 

 cilia early, as in the cases of Serpula and other larvaa described 

 below. 



Tlie atrochal larvge are not common. The following history of an 

 Eunicidan larva (probably Lunibriconereis) from Claparede and Metschiii- 

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



In the earliest stage noticed the larva lias a spherical form, tlie prfe- 

 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 pra?-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 setfe indicating the 

 segmentation are formed successively in the posterior ring-like area free 

 from cilia. The cilia disappear after the formation of two segments. 



In Lunibricus, the embryo of which ought perhaps to be grouped witli 

 tlie atrocha?, the cilia (Kleineiiberg) cover a ventral tract of epiblast 

 between the two mesoblastic cords, and are continued anteriorly to form 

 a circle round the mouth. 



The monotrochal Iarva3 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 (1) 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 ancestral one. 



Their mode of development is well illustrated by the case of Terebella 

 nebulosa (vide Milne-Edwards, No. 347). The embryo is at first a nearly 

 spherical ciliated mass. One end slightly elongates and becomes free from 



