Phylum IV. ECHINODERMATA 



The Echinoderms are animals with primarily a radial (usually pentamerous) 

 and secondarily more or less bilateral symmetry, which were formerly included 

 with the Coelenterates under the general category of Radiata ; but were 

 recognised by Leuckart as the representatives of a distinct animal type. 

 Recently it has been suggested by two authors, working independently, one 

 from a study of comparative embryology (Patten), and the other from 

 evidence furnished by the adult anatomy (A. H. Clark), that the Echinoderms 

 are derived from acraniate crustacean ancestors, through the Cirripedia. 



Echinoderms possess a well -developed, usually pentamerous dermal 

 skeleton, which is composed of calcareous plates, or of minute, isolated, 

 calcareous bodies embedded in the integument, and sometimes also in the 

 walls of many of the internal organs. The exoskeleton may be immovable, 

 or more or less movable, but is very frequently provided with movable 

 appendages (spines, pedicellariae, etc.). The arrangement of both the 

 skeletal parts and the principal organs is so generally pentamerous, that 

 five may be regarded as the fundamental numeral pervading the phylum of 

 Echinoderms. 



Apart from this constitutional difference, Echinoderms are distinguished 

 from Coelenterates by the presence of a true digestive canal, a distinct 

 body-cavity, a vascular system, and a water-vascular apparatus ; by a more 

 perfectly developed nervous system ; and, except in certain Starfishes, by an 

 exclusively sexual mode of reproduction. 



The skeleton of Echinoderms is primarily composed of a series of plates 

 which are situated in the integument, and are covered with living dermal 

 tissue during life of the individual. Although lying near the surface, the 

 plates are strictly internal in position, and are capable of growth or resorption 

 throughout life. Besides skeletal plates, other hard parts may occur, such as 

 spines, pedicellariae, the jaws or so-called " Aristotle's lantern " of Echini, and 

 spicules of the kind found in the tube-feet and some of the internal organs. 

 Certain Crinoids also show a series of calcified convolutions supporting the 

 digestive tube. The calcification of the internal organs is sometimes sufficient 

 to form solid skeletal parts. The plates and other skeletal parts of Echino- 

 derms are composed of open cribriform tissue (Figs. 226, 227), which in the 

 cleaned test of Recent specimens is highly porous. During fossilisation the 

 interstices are commonly infiltrated with lime carbonate, so that the whole 

 structure is transformed into calcite, exhibiting unmistakable rhombohedral 

 cleavage. Each plate, joint and spine of a sea-urchin, star-fish or crinoid 



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