10 NATURAL HISTORY BULLETIN 



When they become columnar or of hour-glass shape, and have 

 the summit covered with a radiating cluster of small slender 

 spinules, they are called paxillge. These are the most typically 

 formed in Astropecten, Luidia, and allied genera, and are char- 

 acteristic of the suborder Paxillosa.^ 



In nearly all the families there are two rows of specialized 

 plates on each side of each ray, known as superomarginal and 

 inf eromarginal plates. These are often the largest and most con- 

 spicuous of all the plates, especially in the Phanerozona, such as 

 Astropecten, Goniaster, etc. In other cases both rows are much 

 reduced in size and are not easily distinguished, as in some of 

 the Asteriidae. Or only the upper row may be reduced and in- 

 distinct, as in Luidia. Rarely one or both rows are abortive in 

 certain deep-sea genera. Secondary intermarginal rows of plates 

 are often interpolated between the marginal rows in some fam- 

 ilies. 



Between the inferomarginals and the adambulacral plates 

 there may be one or more rows of plates called interactinals. The 

 first and most important of these is next the inf eromarginal row, 

 and is called the peractinal row in this article. It may extend 

 nearly or quite to the tip of the Ray. The other rows are called 

 subactinal rows, and may be much shorter and later formed. All 

 these interactinal rows may be lacking in many genera, or they 

 may be rudimentary and spineless. 



They appear later in development than the marginal plates, 

 and the new ones are not formed next the ocular plates, as are 

 the marginals. 



Pedicellariee of one or more kinds are generally present, but 

 are rare or entirely lacking in certain families, such as the Solas- 

 teridce, EchinasteridcE, Pterasteridce, and in some genera of other 

 families. Their uses are imperfectly known. One use it to keep 

 the body free from dirt and parasites. 



These curious organs are not found in other classes of Echino- 

 derms, except in the Echinoidea. In the latter they are com- 

 monly mounted on long stalks and generally have three valves — 

 sometimes four or more or only two. 



In the Asterioidea they are usually sessile, but in the order 



6 For various other special forms and their names, see below, under 

 Phanerozona and Valvulata. 



