SEGMENTED WORMS 



359 



Fig. 880.— Spirorbis. A, Shell. 

 B, Animal, enlarged: e.c, egg- 

 cells within body ; g, gills ; n, 

 kidney tubes {}iephridia] ; op, 

 hollow operculum with eggs [e); 

 st, stomach. 



is not only used to close the mouth of its dwelling, but is also 

 hollowed out to serve as a brood-pouch. 



Most marine Bristle-Worms hatch out in 

 the larval form known as a Trochosphere, 

 and this is also the case with Polygordius, a 

 much elongated and very slender pink form, 

 which is the largest representative of the 

 closely -allied group of Simple Segmented 

 Worms (Archiannelida). Being a typical 

 case, it may well be taken to illustrate the 

 kind of life -history which is usually found 

 among marine Annelids. 



The form of a Polygordius larva (fig. 881) 

 has been compared to that which would be 

 presented by two blunt cones placed base to 

 base. The broader and more rounded of 

 these is the head-lobe (pros^ommm), which 



is kept uppermost during swimming, although at the front end 

 of the body. At its base is a double row of long cilia, by which 

 locomotion is effected. The lower 

 and smaller cone narrows somewhat 

 abruptly to a blunt point, which we may 

 consider to be the tail end of the larva. 

 On one side of the body a mouth is 

 present which marks the under or 

 ventral surface. It leads into a short 

 gullet that is succeeded by a dilated 

 stomach, and this again passes into a 

 short intestine opening at the tail end. 

 The ectoderm at the top of the head- 

 lobe is thickened into a plate, which 

 serves as a brain, and two eye-spots 

 are embedded in it. A circlet of com- 

 paratively small cilia surrounds the 

 body a little way below the mouth. 

 With the internal changes that take 

 place in the larva we are not here con- 

 cerned, but it gradually becomes trans- 

 formed into the adult by elongation of the tail end, and division of 

 this region into rings or segments. It soon becomes apparent that 



Fig. 881. — Trochospheres of Polygordius, 

 enlarged. A and B, Younger and older stages; 

 BR., brain; c, ciliated band ; G, gullet; ST., 

 stomach; int., intestine; K, larval kidney. 



