SEGMENTED WORMS 



359 



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. 88 1) p . g 88o _ Spirorbis . A> shell 

 has been compared to that which would be Animal, enlarged: e.c., egg - 



A cells 



, - . - 111 body; g, gills; n, 



presented by two blunt cones placed base to kidney tubes (nephridia}-, op, 



/T^I -i i i i i r hollow operculum with eggs (e}; 



base. I he broader and more rounded of sf , stomach. 



these is the head-lobe (prostomium), 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. 



