450 The Animal Kingdom 



THE CLASS ARCHIANNELIDA 



The members of this class show the typical annelid characters in 

 a somewhat modified form. It is not entirely clear as to whether these 

 characters should be considered as primitive or as degenerated from 

 more highly specialized species. Usually these worms are small in 

 size. They live along the coastal areas all over the world. One of the 

 commonest genera of this class is the genus Polygordius. 



Polygordius. — These slender-bodied worms vary in length from 

 30 to 100 mm. Externally the segmentation is indistinct except in 

 the anal region ; internally it is indicated by septa. There are no setae 

 or parapodia. The anterior end of the worm bears a pair of sensory 

 tentacles and two ciliated grooves which are probably also sensory. 

 The anal segment bears papillae and projections. Several pairs of ne- 

 phridia are present; the nerve cord is single and has no ganglia and 

 is not separated from the epidermis by mesodermal elements. A 

 circulatory system consisting of several blood vessels is also present. 



In this genus, the sexes are separate and the gonads develop in 

 the posterior segment without special reproductive ducts. The eggs 

 hatch into a typical trochophore larva. 



THE CLASS POLYCHAETA 



With the exception of a few fresh-water forms, the numerous 

 species of this class are marine. They occur from intertidal zone to 

 the extreme depths ; some are pelagic and may at certain times be 

 seen swimming at the surface of the ocean in vast numbers. A well- 

 known genus of this group that occurs on both coasts of this country 

 is the sandworm or clamworm, Neanthes (formerly Nereis). This worm 

 lives in the lower portion of the intertidal zone and is often used as 

 bait by coastal fishermen. 



The Anatomy of Neanthes. — The elongate, somewhat flattened, 

 segmented body of Neanthes (Fig. 146,5) is composed of a varying 

 number of segments. This number may be as high as 200 and the 

 length may be as much as 40 cm. As the worm grows, new segments 

 are added just anterior to the anal one. The anteriormost segment is the 

 peristomium which bears four pairs of tentacular cirri, two ventral and 

 two dorsal. Anterior to the peristomium is the prostomimn. Laterallv 

 this bears a pair of stout palps, dorsally there are two pairs of eyes, and 



