172 MARINE INVERTEBRATES 



hair-like appendages, called cirri. These are sense-organs and also 

 have a respiratory function. The cirri too vary in shape in dif- 

 ferent genera. Generally they are hair-like, but sometimes con- 

 ical ; again they are broadened into scales, as in the Aphroditidce. 



The body-cavity in chaatopod worms is divided into a series of 

 chambers or segments. These divisions seem on the exterior like 

 constrictions, and give the body the appearance of a series of rings. 

 The internal organs are repeated in each segment, so that each of 

 the latter contains a portion of the alimentary canal, a pair of 

 nephridia, a pair of nerve-ganglia, and blood-vessels which connect 

 the main blood-vessels running along the dorsal and ventral sur- 

 faces. The alimentary canal runs through the body, but is con- 

 stricted at each joint. The nephridia are curved tubes, which 

 are excretory organs, opening to the outside and carrying off 

 the waste products which have passed into the fluid of the body- 

 cavity. 



The chaetopod worms have a well-developed nervous system, 

 beginning with a two-lobed ganglion forming a brain and head, 

 then extending through the worm in a double ventral chain and 

 series of ganglia. They have also a blood- vascular system. The 

 blood is either colored or colorless. Sometimes it. is a bright red 

 or green, and is often visible through the body-wall. The circu- 

 lation is effected, not by means of a heart, but by wave-like (peri- 

 saltic) contractions of the dorsal blood-vessel. The body is cylin- 

 drical, but in many cases is somewhat flattened. There are two 

 layers of muscles, one of which encircles the body, while the other 

 extends parallel with its length. Respiration is effected by gills, 

 which may be simple, hair-like appendages to the parapodia, or 

 branched, or comb-like in form. Sometimes they are confined 

 to the middle segments, as in Arenicola ; sometimes to the seg- 

 ments near the head, as in Tubicola ; or they may be extended 

 over the whole dorsal surface. 



SUBCLASS POLYCKffiTA 



C" Many -bristled ") 



The polychaete worms have one or two well-developed parapodia, 

 or limbs, on each side of every segment of the body, and on each 



