EMBEYOLOGY OF THE ANNELIDANS. 275 



endowed with lively motion. They are extremely unlike their parent 

 both in form and structure, being short, oval, cylindrical, and devoid of 

 segmentation, furnished with a circle of long cilia around the centre of 

 the body, but otherwise without external organs. The portion of the 

 body situated anterior to the ciliary circle is somewhat narrower than 

 the hinder one, and bears two eyes : this is the head ; and the young one 

 always swims with this extremity in front. Frequently these young 

 animals revolve, during swimming, around their longitudinal axis. Their 

 sight is distinctly developed ; for they avoid each other with adroitness, 

 and always swim towards the light. The time from the extrusion of the 

 young to the laying of the eggs may probably amount to a couple of 

 weeks. 



(706.) Many interesting particulars relative to the development of 

 various genera belonging to the class under consideration have been 

 ascertained by Milne-Edwards*. In the Terebellce (fig. 136), according 

 to the observations of this distinguished naturalist, the young, on leaving 

 the egg, have no resemblance whatever to the adult animal, insomuch 

 indeed that it would be difficult to guess, a priori, the class to which 

 they really belonged. On their first appearance upon the stage of active 

 existence they might be mistaken for the ciliated larvae of certain Polyps 

 or Medusa?, presenting no traces of the annulose type of structure 

 (fig. 136, l) : in the course of a short time, however, their bodies be- 

 come elongated, and they begin to assume somewhat of a bilateral or 

 symmetrical form, the body of the young Terebella becoming distin- 

 guishable, divided into four zones or rudimentary segments, the pos- 

 terior of which is still provided with a ciliary apparatus (fig. 136, 2). 

 Shortly after this, a fifth ring (fig. 136, 3, cT) begins to make its ap- 

 pearance in the space situated between the penultimate and terminal, 

 and rudiments of a mouth and alimentary canal become distinguishable. 

 The growth of the young Annelidan now begins to advance rapidly ; and 

 its body is rendered more worm-like as new segments are progressively 

 added to its length, these all successively making their appearance in 

 the space between the last-formed ring and the anal or terminal joint 

 of the body ; so that the relative position of the newly developed seg- 

 ments is precisely in accordance with their respective ages, except in 

 the case of the last segment, which is persistently terminal. Meantime 

 the larva ceases to be, as it was at first, completely apodous : simple 

 subulate setae, supported upon minute fleshy tubercles, begin to show 

 themselves on both sides of the body, the development of these loco- 

 motive appendages being accomplished in the same order of sequence as 

 that of the segments, namely from before backwards. 



(707.) At this period of their growth the young Terebellce present 

 the appearance of minute subcylindrical worms (fig. 136, 4), and the 



* " Kecherches Zoologiques faites pendant un Voyage sur les Cotes de la Sicile, par 

 M. Milne-Edwards," Ann. des Sci. Nat. for 1844. 



