CH^ETOPODA. 



327 



FlG. 144. POLYGOR- 

 DIUS LARVA. (From 

 Alex. Agassiz.) 



portion, which by its subsequent elongation gives rise to the whole 

 segmented portion of the future Polygordius. 



At the front end of the prae-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 permanent body cavity formed between the two layers 

 of the mesoblast. It is doubtful when the replace- 

 ment takes place in the head. It probably does so 

 very early. The mesoblast is present in the usual 

 form of two bands (me.p] (germinal streaks), which 

 are anteriorly continued into two muscular bands 

 which pass through the embryonic body cavity to 

 the front end of the prae-oral lobe. Another pair of 

 contractile bands passes from the same region of the 

 prse-oral lobe to the oesophagus. 



There is no trace of the ventral nerve cord. The 

 most remarkable organ of the larva is a paired excre- 

 tory organ (npti) discovered by Hatschek. This is a 

 ciliated canal with at first one and subsequently 

 several funnel-shaped openings into the body cavity in front and an 

 external opening behind. It is situated immediately anterior to the lateral 

 band of mesoblast, and is parallel with, and dorsal 

 to, the contractile band which passes off from 

 this. It occupies therefore a position in front 

 of the segmented region of the adult Polygordius. 



The changes by which this peculiar larval form 

 reaches the adult condition will be easily gathered 

 from an inspection of figs. 143 148. They con- 

 sist essentially in the elongation of what has been 

 termed the abdominal region of the body, and the 

 appearance of a segmentation in the mesoblast ; 

 the segments being formed from before backwards, 

 and each fresh segment being interpolated between 

 the anus-bearing end of the body and the last 

 segment. 



As the hind portion of the body becomes 

 elongated the stomach extends into it, and gives rise to the mesenteron 

 of the adult (figs. 143, 144, and 145). For a long time the anterior spherical 

 dilated portion of the larva remains very large, consisting of a prae-oral 

 lobe and a post-oral section. The two together may be regarded as con- 

 stituting the head. 



At a comparatively late stage a pair of tentacles arises from the front 



FIG. 145. POLYGOR- 

 DIUS LARVA. (From 

 Alex. Agassiz.) 



