CLARK: THE ECHINODERMS OF PERU. 343 



the region. Several species reach a large size, and at least one (Strongy- 

 locentrotus albus) is of considerable importance as an article of food. Of 

 the twelve species, five are characteristic of the region, though one of these 

 has been taken at the Galapagos Islands. Six of the remaining species are 

 Panamic forms, while the seventh is characteristic of the southeastern 

 Pacific Islands and its occurrence on the American coast is exceedingly 

 doubtful. The twelve species represent nine genera, six of which occur 

 in the West Indian region, two are characteristically Pacific, and one 

 (Tetrapygus) is peculiar to the Peruvian region. With the exception of 

 Strongylocentrotus albus, which probably has come up the coast from the 

 south, all of the species have doubtless come from the north, and it is 

 interesting that no fewer than four of them have differentiated into well- 

 characterized forms. 



The shell, or more properly test, of a sea-urchin to be well examined should 

 be dry, and partly or wholly cleaned from the spines which cover it. It is made 

 up of vertical columns of plates ; in all living species there are twenty of these 

 columns, and in most sea-urchins the plates are so firmly united with each other 

 that the test is hard and unyielding. At the upper end of each pair of columns there 

 is a single plate, and these ten plates form in the " regular echini " a ring around 

 the periproct, the field in which lies the anus, while in " irregular echini " they 

 form a solid group, the periproct lying outside of them, usually on the lower sur- 

 face of the test. When the periproct lies outside of them, they form the abactinal 

 system, but when the periproct is within, it is also included in the term " abactinal 

 system." Examination of the columns of plates which make up the test will 

 show that these columns are not only arranged in ten pairs, but that the plates 

 of alternate pairs are perforated for the passage of tube-feet; there are thus five 

 double columns of perforated plates (i. e. with tube-feet) called the ambulacra, 

 and alternating with them five double columns of unperforated plates, the inter- 

 ambulacra. The perforations in the ambulacral plates are in pairs and these 

 pore-pairs may be arranged in a vertical series on each side of an ambulacrum. 

 Often however they are arranged in oblique arcs of three or more in each plate. 

 In the abactinal system the five plates at the upper ends of the ambulacra are 

 called oculars and the five at the ends of the interambulacra, genitals. Usually the 

 genitals being larger than the oculars are readily distinguishable by their size, and 

 in the great majority of sea-urchins three or more oculars lie outside the genitals 

 (i. e. away from the anus). But the exact arrangement of the plates of the abac- 

 tinal system shows great diversity in different families and genera. One of the 

 genitals, usually easily distinguished by its larger size, is perforated with nu- 

 merous small pores. This genital is known as the madreporic plate. The ambu- 

 lacrum at the left side of the madreporic plate is considered anterior, i. e. it marks 

 the anterior end of the animal. The test itself is more or less fully covered with 



