308 



ECHINODERMA 



cloaca. Many zoologists regard them as defensive structures because of 

 their sticky nature and because they can be cast out through the anus. 



The oesophagus is usually surrounded by a ring of five radial and five inter- 

 radial plates which serve as points of attachment for the longitudinal muscles. 

 Just behind it lie the ring canal, ring nerve, and the ring of the blood system, 



each giving off five radial branches which run 

 inside the muscular sac of the body. From 

 the beginning of the radial canals (rarely, as in 

 Synapta, from the ring canal) tubes extend out- 

 ward to form the extremely sensitive retractile 

 tentacles which surround the mouth, and which 

 either branch (Dendrochirotas) or bear frilled 

 shield-shaped extremities (Aspidochirota;). A 

 single Polian vesicle is usually present, and 

 the occasionally branched stone canal connects 

 (except in the Elasipoda) with the ccelom. 



FIG. 309. Transverse section 

 of Holutliuriatub'ulosa (after Lud- 

 wig) . (/, digestive tract ; db, dorsal 

 blood-vessel; g, gonad duct; h, 

 skin; /;;/, longitudinal muscles; 

 Iw, left branchial tree; m, mesen- 



Blood-vessels going from the vascular ring 



tenes; r l ,r 2 , ambulacra! complex 

 of bivium (ambulacral vessel) and 

 nerve; r 3 -r 3 , same of trivium; rw, 

 right branchial tree. 



form rich anastomoses on the alimentary canal. 

 Only a single gonad (or a pair of united 

 gonads) occurs. This consists of numerous 

 tubules which open interradially, usually near 

 the mouth. 



The regenerative powers of these animals 

 are of interest. In unfavorable conditions 

 they void the whole viscera and yet may live 

 and reproduce the lost parts. In certain 

 species are found a few parasites. One or 



two harbor a small fish (Fierasfer) in their cloaca and branchial trees, while 

 parasitic snails (Entoconcha, Entocolax, Entcroxcnus) live in several species and 

 a mussel, Entovalva mirabilis, in another. 



Order I. Actinopoda. 



Radial canals present, sending branches to the tentacles and ambulacra 

 when present. Divided into Pedata, with ambulacra, and Apoda without. 

 The PEDATA include the HOLOTHURID^E with peltate tentacles, (Aspido- 

 chirotae). Holothuria* in warmer waters, one species forming the trepang of 

 Chinese markets. Of the forms with branched tentacles (Dendrochirota?) are 

 the CUCUMARIID.E, Citcumaria* Psolus* Thyonc* The deep sea ELASIPODA 

 have statocysts and peculiar dorsal ambulacral processes. The APODA are 

 represented by Caitdina* and Molpadia* 



Order II. Paractinopoda. 



No radial canals nor ambulacra. Tentacular canals arising from ring 

 canal. Myriotrochw* Synapta* with statocysts, Oligotrochus* In Pelago- 

 thuria the anterior end is expanded to a disc with tentacular processes, used in 

 swimming, like the bell of a medusa. 



Summary of Important Facts. 



i. The ECHINODERMA share the radiate structure with the 

 Coelenterata, but differ from them (a) in the numerical basis of the 

 symmetry (five) ; (b) in that, as embryology shows, they have descended 

 from bilateral forms. 



