256 



CHAETOGNATHA THROUGH LOWER CHORDATA: 



lateral type which became attached to the substrate; 

 certain of the fossil attached forms, which no longer 

 exist, do show bilateral symmetry. In the evolution of 

 attachment, radial symmetry evolved as an adapta- 

 tion to sedentary life. Later, most modern echino- 

 derms lost their stalks and again became free, but re- 

 tained their radial symmetry. Today, most modern 

 echinoderms are still radially symmetrical, but there 

 are sea cucumbers and sand dollars, which, in taking 

 up a crawling life, seem to be selecting for a second- 

 ary bilateral symmetry. 



The Phylum Echinodermata possesses several un- 

 usual features. The water-vascular system is found 

 only in echinoderms. Basically, it consists of a ring 

 anteriorly about the digestive tract and a vessel for 

 each of the animal's arms or rays. In most echino- 

 derms the water-vascular system is used in locomo- 

 tion with the tube feet. Changes in water pressure in 

 areas associated with tube feet cause movement of 

 the tube feet. The skeleton is truly internal (endo- 

 skeleton) like our own. As in other coelomate ani- 

 mals, the gonads of echinoderms arise from the 

 lining of the coelom, but in echinoderms the relation- 

 ship is different. In other coelomates, the eggs at 

 least are shed into the coelomic cavity before leaving 

 the body. Here, the sex cells traverse ducts directly to 

 the exterior without entering the coelom. Fertiliza- 

 tion occurs free in the water. In general, the sexes 

 are separate. The so-called blood system of echino- 

 derms is also very strange. There is no pumping or- 

 gan and the vessels are filled with a jelly-like material. 

 Wandering amoeba-like cells of various kinds move 

 through this blood system and out into the tissues. 

 Some of these cells pick up insoluble wastes, wander 

 through the body wall to the exterior, and there dump 

 the wastes. Respiration and excretion are generally 

 accomplished by means of thin-walled, coelomic ex- 

 tensions that can be projected through the body wall. 

 Gas exchange takes place by dififusion from sea water 

 to the coelomic fluid inside. The sea cucumbers have 

 a special "breathing" mechanism, the respiratory 

 tree. 



Subphylum PELMATOZOA (Pelmatozoans) 



Diagnosis: attached during part or all of life by the 

 end opposite the mouth, attachment either direct or 

 by a stalk; both mouth and anus are on the upper 

 surface; body in a cup-like skeleton (calyx); five 

 classes, four entirely extinct (Heterostelea, Cystidea, 

 Blastoidea, and Erisasteroidea); Cambrian to Recent. 



CLASS CRINOIDEA (Sea Lilies and Feather Stars) 



Diagnosis: calyx symmetrical; arms five, branch- 

 ing at base; tube feel lack suckers, tentacle-like, food- 

 collecting (Figure 15.1). 



Figure 15-1. Antedon, a feather star: left, a free-living adult; lower 

 right, on attached larva, which resembles the opproximately 80 species 

 of adult sea lilies. (Redrawn from Clark.) 



The crinoids, known as sea lilies (attached) or 

 feather stars (free-living), are an abundant group of 

 mostly deep-sea animals. Most, if not all, are filter 

 feeders of microscopic organisms; some may be de- 

 tritus feeders. The young adult grows attached by a 

 stalk to the substrate. Sea lilies remain attached 

 throughout life, while feather stars break loose from 

 the stalk, retaining a portion of the stalk and attach- 

 ment organs. The latter group can creep along the 

 substrate or swim by movement of the arms. This 

 ability to move gives the feather stars an advantage if 

 local environments become unfavorable, but a dis- 

 advantage if ocean conditions move them from a 

 favorable habitat. 



Subphylum ELEUTHEROZOA (Eleutherozoans) 



Diagnosis: free-living, stalk-less; anus, if present, 

 on side opposite mouth; structure regularly five- 

 armed, or -rayed; live classes, only one entirely ex- 

 tinct (Ophiocistioidea); Cambrian to Recent. 



