CHsETOPODA. 



271 



FIG. 144. POLYGOR- 

 DIUS LARVA. (From Alex. 

 Agassiz.) 



usual form of two bands (nte.p) (germinal streaks), which are anteriorly 

 continued into two muscular bands which pass through the embryonic 

 body cavity to the front end of the prse-oral lobe. Another pair of con- 

 tractile bands passes from the same region of the prse-oral lobe to the 

 O3sophagus. 



There is no trace of the ventral nerve cord. The most remarkable 

 organ of the larva is a paired excretory organ (nph) discovered by Hatschek. 

 This is a ciliated canal with at first one and sub- 

 sequently 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 Poly- 

 gordius. 



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 meso- 

 blast ; 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 prse-oral lobe and a post- 

 oral section. The two together may be regarded 

 as constituting the head. 



At a comparatively late stage a pair of ten- 

 tacles arises from the front end of the prse-oral 

 lobe (fig. 146), and finally the head becomes rela- 

 tively reduced as compared with the body, and 

 gives rise to the simple head of the fully formed 

 worm (fig. 148), The two ciliated bands disappear, 

 the posterior vanishing first. The ciliated band 

 at the hind end of the body also atrophies ; 



and just in front of it the ring of wart-like prominences used by the 

 adult to attach itself becomes developed. 



FIG. 145. POLYGOR- 

 DIUS LARVA. (From Alex. 

 Agassiz.) 



FIG. 146. 

 Agassiz. ) 



POLYGORDIUS LARVA. (From Alex. 



