138 



ECHINODERMATA 



^ I 



means of the tube feet. The Holothuroidea utilize their tentacles 

 in movement. The brittle stars move by lateral contractions of 

 their arms. The Echinoidea move by means of spines and a few 



tube feet. 



Physiology. — In the sea urchin we have the first instance of 

 masticatory structures in the invertebrates. The five teeth of the 

 " Aristotle's lantern " are extremely powerful. The digestive 

 system is extremely efficient as seen in the starfish. (See p. 132.) 

 Respiration is carried on by the dermal branchiae. Circulation is 

 not well developed and perhaps is not needed with the complicated 

 water vascular system. The body fluid, hydrolymph or blood, is 

 similar to the fluid in the water vascular system, but is richer in 

 albumen. In the sea cucumber, Thyone, the hemoglobin occurs 

 in small, very numerous corpuscles. Excretion is carried on by the 

 mouth, the dermal branchiae and the intestine. 



Reproduction. — Well-developed gonads are present and sexes 

 are separate in many of the Echinoderms. Parthenogenesis occurs 

 frequently. (See p. 134.) 



Behavior. — The Echinoderms have well-developed reactions to 

 stimuli of touch, light and temperature They also have primitive 

 olfacto-gustatory sense. 



Embryonic Development of the Echinodermata. — The eggs of 

 Echinoderms divide into 2, 4, 8, 16, 32, 64 cells, each finally be- 

 coming a blastula^ then a gastrula^ and finally a larval stage. The 

 larvae are bilaterally symmetrical, with an aUmentary canal from 

 which later bud two coelomic sacs. These form the body cavity and 

 the water-vascular system. The larvae of the different classes 

 vary somewhat in structure. Bilateral symmetry is lost in the 

 adults except as it is retained slightly in the Holothuroidea. 



Importance of Echinodermata in Biological Research. — During the 

 past thirty years, we have seen increased utilization of invertebrate 

 forms in research at our marine laboratories. The starfish and the 

 sea urchin have been the types used in numerous chemical and 

 physiological studies, far-reaching in their significance. Mathews, 

 Child, J. Loeb, Tenn/nt, F. Lillie, R. Lillie, Just, Heilbrunn, Glaser, 

 Sampson, Woodward, and their associates, have been especially 

 active in these studies.^ 



1 See the Wistar Institute Bibliographic Service, and the Journal, Biological 

 Abstracts, for references. Also refer to paper by D. H. Tenn^nt, 1929, Studies in 

 experimental embryology based on sea urchin eggs. Sc. Mon., vol. 29, no. 2 (167), 

 pp. 1 17-124. 



