THE GROUPS OF ANIMALS 269 



Class III. Echinoidea. Free-living, pentamerous echinoderms without arms; 

 test composed of calcareous plates bearing movable spines. Sea urchins, sand dollars. 

 (Figs. 76, 225C, D.) 



Class IV. Holothurioidea. Free-living, elongated, soft-bodied echinoderms with 

 muscular body wall and tentacles around mouth. Sea cucumbers. (Fig. 225E.) 



Class V. Crinoidea. Sessile echinoderms with five arms generally branched with 

 pinnules, aboral pole usually with cirri, sometimes with jointed stalk for attachment 

 to substratum. Feather stars, sea lilies. 



Phylum 7. Annelida. — These are the true worms, as distinguished 

 from Phyhim 4 and Phyhmi 5 whose members are called flatworms and 

 roundworms. The annelids are triploblastic, bilaterally symmetrical 

 animals, with elongated body divided into segments. The segmentation 

 is internal as well as external, for thin membranes divide up the body 

 cavity or coelom. Corresponding with these segments, many of the 

 internal organs are repeated in most of the segments, while some are 

 repeated in only a few of the segments. The excretory organs, which 

 are nephridia (page 135), occur in most segments; the nervous system, 

 which is chiefly a long cord near the ventral side, typically has a ganglion 

 and nerves in most of the segments; and the main blood vessels give off 

 branches in each segment. Spiny projections or setae are common aids 

 to locomotion. 



Some of the annelids are hermaphroditic but do not fertilize their 

 own eggs. Some of them (Fig. 226) also reproduce by budding or unequal 

 fission. Many of them have remarkable powers of regeneration if cut 

 into pieces. Some curious results are obtained by cutting off the head 

 end of an earthworm; at certain levels the head structures are regenerated, 

 while if cut at other levels a tail is developed in place of a head. 



Despite the large size which many annelids attain, some of the larger 

 ones respire only through the general body surface. Some others, no 

 larger, have branched or filamentous gills which greatly increase the area 

 through which oxygen is absorbed. 



Some theoretical interest attaches to the larval stage of many marine 

 annelids, which is known as a trochophore. It is pear-shaped or nearly 

 spherical, with a circle of cilia around its equator. Similar larvae are 

 found among the clams and snails, and adult rotifers may have roughly 

 the same shape. Many biologists have considered that some relation- 

 ship among these phyla is indicated by the trochophore larva or trocho- 

 phorelike adult form. 



Among the services to man performed by annelids may be mentioned 

 the comminution and constant overturning of the soil by earthworms. 

 These animals eat the soil, for whatever organic matter it may contain, 

 and eject it from their digestive tracts. In making their burrows, 

 much soil is brought to the surface from below. The burrows also leave 

 the soil porous. Some annelids are also used for human food, notably 



