618 ECHINODERMATA 



circulation by the cilia which line the inner surface of the spaces and 

 vessels. 



The sexes are separate in the majority of echinoderms, the males and 

 females being more or less alike in appearance. The genital organs are 

 simple in structure. Except in the holothurians and some sea-urchins, 

 they consist of five gonads or pairs of gonads, which open separately to 

 the outside. In the holothurians, they consist of a branched gonad which 

 opens to the outside near the mouth; this gonad may be homologous to 

 the axial organ of other echinoderms. No accessory genital glands or 

 organs are present among echinoderms, and the sperm and ova are 

 usually thrown into the surrounding water, where fertilization takes 

 place. 



Echinoderms have great regenerative powers; an arm broken off is 

 replaced, and a holothurian may regenerate its digestive tract and other 

 viscera if these are voided when the animal is irritated. Certain star- 

 fishes and brittle-stars reproduce asexually, by transverse division. 



Distribution and Habits.— Ytchinoderms are inhabitants of the sea 

 bottoms. They are found in all parts of the globe, the various species 

 ranging from the shore line to the greatest depths. A few forms can 

 swim, but the greater number creep slowly about, feeding on small animals 

 and plants, organic matter in the sand and mud, or seaweeds. 



History.— heiwkart, in 1847, first recognized the Echinodermata as 

 one of the main subdivisions of the animal kingdom. The name origi- 

 nated with Klein, who, in 1734, applied it to the sea-urchins. Lamarck 

 added the starfishes and Cuvier the holothurians to the group, which 

 formed a class under the Zoophyta or Radiata. Leuekart's great service 

 consisted in showing that Cuvier's Radiata formed an assemblage of 

 animals which are without natural relationships and in splitting it into 

 the Ccelenterata and Echinodermata, the former being very primitive ani- 

 mals without a distinct body cavity or a digestive tract, both of which are 

 possessed by the latter. 



The Echinodermata contain about 4,000 species, grouped in five 

 classes. 



Key to the classes of Echinodermata: 



«! Arms present. 

 6i Arms with small branches called pinnules ; oral surface directed upwards. 



1. Crinoidea 

 63 Arms without pinnules ; oral surface directed downwards. 

 Oi Oral surface of arm with a deep longitudinal (ambulacral) groove. 



2. ASTEROIDEA 



Cj Ambulacral grooves absent 3. Ophiuroidea 



Oj Arms absent. 



61 Body covered with movable spines ; no oral tentacles 4. ECHINOIDEA 



6, Body without spines ; oral tentacles present 5. Holothurioidea 



