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 Aphroditide. 
The body-cavity in cheetopod 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 cheetopod 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 gitls, 
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 POLYCHATA 
(“ Many-bristled") 
The polychaete worms have one or twe well-developed parapodia, 
or limbs, on each side of every segment of the body, and on each 
