ECHINOIDEA 641 



The sexes are separate. The gonads are usually five in number and 

 lie in the upper part of the body cavity, opening to the outside through 

 the genital pores. Each is joined by a cord with an apical ring which, 

 as in the starfish, is connected with the axial organ. The genital products 

 are thrown into the sea water; the larva is similar in form to that 

 of the Ophiuroidea, and is also called a pluteus. 



Distribution and Habits. Sea-urchins occur in all seas, being most 

 numerous in the neighborhood of the coast, although many deep-sea forms 

 occur. They move slowly about by means of the ambulacral feet and the 

 spines, and live on small forms of life and organic remains of all kinds; 

 many species pass large quantities of sand and mud through the intestine. 

 Some sea-urchins hollow out excavations in rocks, in which they live. 

 Certain species, in France and Italy and other countries, are used for 

 food, the genital organs being eaten; over 100,000 dozen sea-urchins are 

 yearly brought into the fish markets of Marseilles. 



History. Aristotle was acquainted with several species of sea-urchins 

 and the name Echinus originated with him. He was also acquainted with 

 certain features of their anatomy, and mentions the similarity in appear- 

 ance of the dentary apparatus to a lantern. The first attempt in more 

 recent times to classify them was made by Klein (1734). The knowledge 

 of sea-urchins did not, however, increase rapidly until the following cen- 

 tury. In 1816 appeared Tiedmann's important works on their anatomy, 

 and in the fourth and fifth decades of the century the foundations of the 

 modern classification were laid by Desor and Louis Agassiz, while the 

 metamorphosis was studied by J. Muller. In modern times A. Agassiz, 

 H. L. Clark and R. T. Jackson have done the most to extend the knowl- 

 edge of the group, especially of the American forms. 



Modern Echinoidea contain 4 orders, with about 500 species. More 

 than 2,000 fossil species 'are known. 



Key to the orders of Echinoidea: 



! Without peristomal gills; peristome with plates; mouth central; anus 



apical 1. CIDABOIDA 



a, With peristomal gills. 

 &! Dentary apparatus present ; mouth central. 



GI Anus apical 2. CENTRECHINOIDA 



c, Anus in posterior interradius ; test depressed or discoidal. 



3. CLYPEASTBOIDA 

 & 2 Dentary apparatus wanting ; anus not apical 4. SPATANGOIDA 



ORDER 1. CIDAROIDA. 



Sea-urchins which have no peristomal gills; ambulacral areas nar- 

 row and inter-ambulacral areas broad, and both continued on the peri- 

 stome to the mouth; spines large and long: 1 family, with about 60 

 species, mostly in the warmer seas. 



