326 



THE LARVAL FORM. 



tions, though in some instances it is constant through large 

 groups. In Chaetopods there is a widely distributed prae-oral 

 ciliated band, which is similarly placed to the ring constantly 

 found in the larvae of Molluscs, Rotifers, etc. In many of these 

 forms the band is practically double, the opening of the mouth 

 being placed between its two component rings (vide fig. 142). 

 The best introduction to the study of the Chaetopod larval forms 

 will be the history of the changes of a typical larval form in 

 becoming converted into the adult. 



For this purpose no better form can be selected than the interesting 

 larva of Polygordius (vide Agassiz, No. 332, 

 Schneider, No. 352, and Hatschek, No. 339), 

 which was first discovered by Loven, and 

 believed by him to be the larva of an ordinary 

 Chsetopod. Its true nature was determined 

 by Schneider. 



At a very young stage the larva has the 

 form (fig. 142) of a flattened sphere, with a 

 small conical knob at the posterior ex- 

 tremity. 



At the equator are situated two parallel 

 ciliated bands 1 , between which lies the ven- 



me.p 



nph 



an 



FIG. 142. POLYGORDIUS 

 LARVA. (After Hatschek.) 



m. mouth ; sg. supra-ceso- 

 phageal ganglion ; nph. neph- 

 trally placed mouth (;). The more conspicu- ridion ; me. p. mesoblastic band ; 



ous ciliated band is formed of a double row an - anus ; ol - stomach. 



of cilia, and is situated in front of the mouth. The thinner ciliated band 



behind the mouth appears to be absent in the American species. 



The mouth leads into an oesophagus, and this into a globular stomach 

 (ol), which is continuous with a rectum terminating 

 by an anus (an) placed at the hind end of the 

 posterior conical knob. The whole alimentary 

 tract is ciliated. In the American form of larva 

 there is a ring of cilia round the anus, which is 

 developed at a somewhat later stage in the form 

 observed by Hatschek. 



The position of the ciliated bands and the 

 alimentary tract enables us to divide the embryo 

 into three regions : a prae-oral region bounded by 

 the anterior ciliated band, a gastric region in 



which the embryonic stomach is situated, and an DIUS LARVA!* (From Alex, 

 abdominal region formed of the posterior conical Agassiz.) 



FIG. 143. POLYGOR- 



1 These two rings are at first (Hatschek) not quite closed dorsally, calling to mind 

 the early condition of the Echinoderm larva; with a prs-oral and post-oral ciliated 



area. 



