270 



THE LARVAL FORM. 



me.p 



nph 



an 



ponent rings (vide fig. 142). The best introduction to the study of 

 the Choetopod 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 deter- 

 mined 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 extremity. 



At the equator are situated two parallel ciliated bands ', between which 



lies the ventrally placed mouth (*). The 

 more conspicuous ciliated band is formed of 

 a double row of cilia, and is situated in front 

 of the mouth. The thinner ciliated band be- 

 hind the mouth appears to be absent in the 

 American species. 



The mouth leads into an resophagus, and 

 this into a globular stomach (ol), which is con- 

 tinuous 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 em- 

 a pra3-oral region bounded by the anterior 

 ciliated band, a gastric region in which the embryonic stomach is situated, 

 and an abdominal region formed of the posterior conical portion, which 



by its subsequent elongation gives rise to the 

 whole segmented portion of the future Poly- 

 gordius. 



At the front end of the prse-oral lobe is situated 

 the early formed supra-cesophageal ganglion (sg) 

 (first noticed by Agassiz) in connection with which 

 is a pair of eyes, and a ramified system of nerves. 

 The ganglion is marked externally by a crown of 

 cilia. 



The larval epidermis bears a delicate cuticula, 

 and is separated by a considerable interval from 

 the walls of the alimentary tract. The space 

 between the two represents a provisional body 

 cavity, which is eventually replaced by the per- 

 manent body cavity formed between the two layers 

 of the mesoblast. It is doubtful when the replacement takes place in the 

 head. It probably does so very early. The mesoblast is present in the 



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

 the early condition of the Echinoderm larvre with a prae-oral and post-oral ciliated area. 



FIG. 142. POLYGORDIUS 

 LARVA. (After Hatschek. ) 



m. mouth; sg. supra-ceso- 

 phageal ganglion; nph. neph- 

 ridion ; me.p. niesoblastic band; 

 an. anus; ol. stomach. 



bryo into three 



regions 



FIG. 143. POLYGOR- 

 DIUS LARVA. (From Alex. 

 Agassiz.) 



