ANNELIDA 



277 



The larvse of Ophryotrocha piierilis (Fig. 127) are Polytro- 

 chce — segmented larv« in the proper sense. They possess a 

 number of segments, each one of which 

 is provided with a ciliated band. In 

 addition, stouter tactile hairs are found 

 at the anterior and posterior ends of the 

 body. The first ciliated band belongs 

 to the head region of the larva. Next to 

 this is situated the mouth-opening, which 

 leads into a large pharynx provided with 

 a chewing apparatus. The intestine 

 extends straight backwards, and opens 

 to the exterior at the end of the last seg- 

 ment. The anus is situated dorsally, in- 

 asmuch as the last segment possesses a 

 pointed process on the ventral side (Fig. 



127;. 



The next stages of development in Oiiliryo- 

 trocha remain much like the larva described, 

 since the new segments formed in front of the 

 anal segment are also provided with bands of 

 cilia. Knob-like parapodia then bud out on the 

 segments, and in them the set« are developed. 

 The number of the segments is considerably in- 

 creased, yet this small Annelid, which never 

 becomes over 2'5 mm. long, remains, as it were, 

 in a larval condition, since the segments retain 

 their ciliation throughout life. Still another 

 ciliated band has been developed on the head, 

 and two small knob-like feelers have arisen there, 

 which bear long cilia, just as do the two cirri 

 which have made their appearance near the unpaired process on the 

 anal segment. The two most anterior segments remain without appen- 

 dages (Claparede und Metschnikoff). 



Fig. 126. — Annelid larva 

 with provisional setae 

 (after Alex. Agasstz, 

 from Balfoük's ComparO' 

 Hue Embryology). 



In Ophryotrocha the ciliated rings surround the entire 

 segment. They appear to be arranged in the same way in 

 Arenicola marina ; in other larvae, on the contrary, they have 

 the form of half-rings only, and are confined to the dorsal 

 or ventral surfaces (Nototrochce and Gasterotrochce) . Noto- 

 trochal larvae are found, for example, in Terehella conchilega, 



