456 ECHINODERMATA ASTEROIDEA chap. 



(c) Pedicellariae. — These are to be looked on as spines of 

 the second order. In Asterina and its allies they are not 

 present, but groups of little spines arranged in twos and threes, 

 each group being attached to a special small plate, are scattered 

 over the aboral surface ; and these on irritation approach one 

 another, and represent the rudiment out of which pedicellariae 

 have been developed. The most perfect form, termed " forci- 

 pulate," in which there is a basal ossicle, is found in Asteriidae, 

 Brisingidae, Heliasteridae, Pedicellasteridae, Zoroasteridae, Stich- 

 asteridae. There are two varieties of forcipulate pedicellariae, 



the " crossed " and the " straight," 

 which have been described on 

 p. 432. In all other cases the 

 pedicellariae are devoid of the 

 basal ossicle, and the two or more 

 spinelets forming the jaws are 

 directly attached to one of the 

 main plates of the skeleton. 



The simplest variety is termed 

 Fig i95.-Differeut forms of pedicel- » pectinate " ; thcse pedicellariae 



lanae (excluding the forcipulate form, ^ ' ^ 



for which see Fig. 186). A, pecti- are composcd of two parallel 



nate; B pectinate; C yalvate ; D, ^^^^,^ ^f g^^j^ -^^g OppOSCd tO 

 pmcer-shaped ; E, alveolate, from the ^ '^ ^ 



side ; F, alveolate, from above, x 10. each Other. They are found in 

 (After siaden.) ^-^^ Archastcridae, and are hardly 



more advanced in structure than the groups of spines found 

 in Asterina. In Leptogonaster and its allies there are pincer- 

 shaped pedicellariae composed of two curved rods articulating 

 with one of the plates of the skeleton, and also " alveolate " 

 pedicellariae, composed of two short prongs which are im- 

 planted on a concave tubercle borne on one of the plates of 

 the skeleton. In the Antheneidae every plate of the ventral 

 surface bears a large " valvate " pedicellaria consisting of two 

 horizontally elongated ridges, which can meet one another. It 

 is possible that valvate pedicellariae have been derived from a 

 pectinate form in which successive spinules of one row have 

 become adherent. 



{d) Ambulacral Skeleton. — In every case, whether spines 

 are developed elsewhere or not, the adambulacral plates bear 

 spines. Where the spines are elsewhere represented by granules 

 {Nardoa and its allies) (Fig. 194, D) the adambulacral spines are 



