TYPE ECHINODERMA. 553 



the so-called odontopliore plates, which are generally small 

 and in many cases covered over by other plates of the oral 

 surface, and lie in the immediate neighborhood of the 

 mouth. At the junction of the oral and aboral surfaces of 

 the disk and arms two series of plates are frequently to be 

 found which from their position are termed the supra and 

 infra-marginals, and, in addition to these, series of plates 

 with definite arrangement are developed in connection with 

 the water vascular system. Thus along each side of the mid- 

 dle line of each arm is a series of plates, which, sloping 

 aborally and towards the axis of the arm, meet to form the 

 floor of an ambulacral groove which extends outwards from 

 the mouth to the extremity of the arm. These are the am- 

 bulacral plates (Fig. 254, A), and each series of them is flanked 

 upon the outer side by a row of adambulacrals (Fig. 254, B} 

 whose number may or may not correspond with that of the 

 ambulacrals. Between the adambulacral series of adjacent 

 arms a series of plates may be interposed upon the oral sur- 

 face of the disk, and to these the name of inter ambulacrals 

 may be applied. 



Spines are very frequently borne by the plates or reticu- 

 lum of the aboral surface, but are usually low and immova- 

 ble, though upon the marginal and adambulacral plates they 

 are very frequently longer, united to the plates by a rudi- 

 mentary articular surface and supplied with muscle-fibres by 

 which they can be moved. In addition to these appendages 

 of the dermal skeleton, others are to be found in the Star- 

 fishes, such, for example, as the ciliated spines found in a 

 few forms, such as Luidia, upon the marginal plates. These 

 spines are small and delicate, and grouped together, their 

 principal peculiarity being that they are covered by an 

 epithelium of high columnar cells which bear strong cilia. In 

 most Starfish also peculiar structures termed pedicellarice are 

 developed in connection with the skeleton, but their descrip- 

 tion may be deferred until the Echiuoids are under discus- 

 sion, in which group they reach a high grade of develop- 

 ment. Peculiar to certain genera of Starfishes, e.g. Luidia, 

 are the paxillw, found principally upon the aboral surface of 

 the body. They consist of small columns of carbonate of 



