THE PENTAMEROUS SYSTEM AND VARIATION. 35 



The Pentamerous System and Variation. 



In echinoderms as a whole the pentamerous system is remarkably constant as a character. 

 In cystoids, however, we may have less than five ambulacra, as in Echinosphaerites, which 

 has three ambulacra, or more than five, as in Caryocrinus, which has many ambulacral parts. 

 In starfishes many genera, as Heliaster and Solaster, have typically more than five areas, and 

 in some that have five, variation is common. In brittlestars more than five areas occur rarely 

 as a specific, and in one instance as a sexual character. In all other groups of echinoderms 

 the pentamerous system is very constant in all genera, and is departed from only as an indi- 

 vidual variation. This variation is quite common in blastoids and crinoids. In Echini 

 amongst Palaeozoic species, no case is known of departure from the pentamerous system, and 

 it is apparently rare in living Echini. Bateson (1894) gives thirteen cases of complete or 

 partial reduction to four-rayed sea-urchins in several genera of post-Palaeozoic fossil or Recent 

 Echini. He also gives two cases of six-rayed urchins, a Galerites and an Amblypneustes, and 

 three cases in which there is a sixth ambulacrum only. Chadwick (1898) described a four- 

 rayed Echinus esculentus Linne, in which, however, there were five teeth. Ritchie and 

 Mcintosh (1908) describe, very carefully, an Echinus esculentus with a partial reduction to 

 four areas. Tower (1901) records an Echinarachnius parma from Woods Hole with one of 

 the ambulacra imperfectly developed, and Hawkins (1909a) an Amblypneustes with two ambu- 

 lacra incomplete dorsally. Osborn (1898) described a four-rayed Arbacia pun£tulata, de Loriol 

 (1883) a partially six-rayed specimen of Stomopneustes, and Ribaucourt (1908) a completely 

 hexamerous Strongylocentrotus lividus. 



I have been fortunate enough to study 71 specimens of Echini with a complete or partial 

 departure from the pentamerous system. Besides being curiosities, these show very interest- 

 ing morphological characters. Sixty were discovered in an examination of about 50,000 

 specimens for the characters of the apical disc, so that the variants from the pentamerous 

 system averaged a httle more than one to a thousand. The variants are partially or completely 

 trimerous, tetramerous, and hexamerous. They are numerically as follows: one Eucidaris 

 tribuloides in 849 specimens, six Arbacia punctulata in 2,329 specimens, two Echinus magel- 

 lanicus in 200 specimens, 38 Strongyloeentrotus drobachiensis in 33,000 specimens, one Strongy- 

 locentrotus eurythrogrammus in 56 specimens, two Toxopneustes variegatus in 1,043 specimens, 

 five Toxopneustes atlanticus in 2,643 specimens, three Tripneustes esculentus in 703 specimens, 

 one Echinometra lucunter in 754 specimens, and one Colobocentrotus atratus in 82 specimens. 



As later discussed, the ocular plates seem to exert a controlling influence in the building 

 up of the corona, as below and in immediate contact with the oculars originate the coronal 

 plates, both ambulacral and interambulacral. In connection with each ocular is developed a 

 whole ambulacrum, and, in addition, a half-interambulacrum on either side. That is, while 



