259 



THE PRICKLY-SKINNED ANIMALS (ECHINODERMATA). 



Position of the Echinwler mata in the Animal Kingdom Characters distinguishing them from the other Radiata Subdivisions 

 of the Group The Limestone Skeleton : its Various Forms The Digestive Tube The Nervous System The Blood- 

 vascular System The "Water- vascular System The Body-cavity and its Contained Corpuscles The Various Forms of 

 Echinoderm -larvae Direct Development of some Echiiioderms in the Southern Seas Distribution of the Group ia 

 Space and Time The Echinozoa Structure and Habits of Starfishes, Ophiurids, Urchins, and Holothurians The 

 Trepang fisheries of the Tropical Seas The Crinoidea Structure and Mode of Life of a Feather-star The Palaeozoic 

 Crinoids, Cystids, and Blastoids. 



THE Marine Invertebrata known as Eckinodermata* owe their name to the prickly nature of their 

 skin, which is usually more or less thickly set with spines and granules of limestone. Such animals 

 are the Sea-urchins, or Sea-hedgehogs (Fig. 14), the Starfishes (Fig. 1), Sand-stars, Brittle-stars 

 (Fig. 11), Feather-stars (Fig. 19), Sea-lilies (Fig. 18), and the Sea-cucumbers (Fig. 17). 



The Echinoclerms constitute one of the three great groups of animals which were 

 associated by Cuvier under the name Radiata. The especial characteristic of this division of the 

 animal kingdom is the arrangement of the various organs of the body in a radiating manner around a 

 central axis, in which the mouth is placed. The 

 bilateral symmetry, so apparent in the Articulata 

 and in the bivalve Mollusca, is not visible at 

 first sight in such animals as a Starfish, a Sea- 

 anemone, or a Jelly-fish. The younger stages of 

 all these creatures, however, exhibit a more or 

 less evident bilateral symmetry (Figs. 2-5, 7) ; 

 and this is sometimes quite distinct in the adult 

 animal (Figs. 1, 11, 14, 19), for a median plane 

 can be found, with the parts of the body which 

 lie on either side of it disposed symmetrically in 

 relation to it. The body of an Echinoderm is 

 also far more complicated in its structure than 

 that of a Polype, or Jelly-fish. The digestive 

 apparatus is entirely shut off from the body- 

 cavity, and there are two separate systems of 

 vessels ramifying through the body, which are 

 either completely independent of the body-cavity, 

 or only communicate with it by special openings. 



On account of these and other striking 

 features in their organisation, the Echiiioderms 

 were removed from the Radiate type by Leuckart, 

 a proceeding which has met with almost unanimous acceptance among European naturalists. But 

 the resemblance between Echinoderm larvse and young Ctenophora is adduced as one amongst other 

 reasons (by some American zoologists) for affirming that the type of Radiates constitutes an inde. 

 pendent division of the animal kingdom, containing three equivalent classes Echinoderms, Jelly- 

 fishes, and Polypes. 



On the other hand, there are considerable resemblances between certain Echinoderms, both larval 

 and adult, and some of the lower worms ; and after Leuckart's removal of the former from the 

 Radiate type, they were thrown, together with the Wheel-animalcules, Tape Worms, Fluke Worms, 

 .fee., into one group the Annuloida,^ the name of which indicated the worm-like (Annelidan) aflinities 

 of some of its members. 



Further investigation has shown, however, that this arrangement is not a satisfactory one, and 

 at the present time the Echinoderms are regarded by most zoologists as forming a distinct primary 

 division of the animal kingdom. Its chief subdivisions are indicated in the following Table, 



1. ASTROPECTEX IRREGULARIS, A COMMON BRITISH STAR- 

 FISH, SLIGHTLY REDUCED. 

 , the madreponte. 



Greek, echinos, hedgehog ; derma, skin. 



t Latin, annulus, a ring ; Greek, sides, form. 



