Characteristics of Echinoidea. cli 



irregular forms Spatangidae and Clypeastridae, they take very 

 various shapes, and amongst them that of the so-called ambulacral 

 gills, which form a rosette of petaloid ambulacra limited to the 

 apical half of the shell. 



The madreporic plate is always to be found at or near the apical 

 pole of the body, and is usually fused with one or more of the 

 genital plates. Five eye-bearing plates alternate radially with the 

 inter-radially placed genital plates. The ambulacral plates lie ex- 

 ternally to the free radial water-vascular trunks, but there are 

 certain internal calcified processes, the so-called ' Auriculae/ which 

 may, as in the genus Chjpeaster (Lam.), Echinanthus (MiiUer), attain 

 a great development, homologous with the internally-placed ambu- 

 lacral ossicles of the Asteroidea. Both the ambulacral and the inter- 

 ambulacral plates are beset with very numerous movably-arti- 

 culated spines of the most varied forms, between which pedicellanae 

 are, as in Asferiae, interspersed. In Spatangidae certain areae, the 

 < semitae/ are occupied by bristle-like appendages, which have club- 

 shaped ends, are strengthened internally by calcareous deposit, and 

 are covered externally with cilia. 



The tentacular corona of the Holothurioidea is represented m 

 Echinoidea by certain largely-developed ambulacral feet placed 

 radially on the innermost circle of the peristomial area, and imme- 

 diately therefore on the edge of the mouth. But in Ecliinidae and 

 Clypeastridae, a complex prehensile masticatory apparatus exists 

 as the so-called ' Lantern of Aristotle,' at the entrance of the diges- 

 tive tract. The teeth of this apparatus are lodged in inter-radially 

 placed alveoli, composed of two main and two accessory pieces, 

 and alternating with radially- placed structures, each consisting 

 in Clypeastridae of one, and in Echinidae of three, ossicles. The 

 elements in the radially-placed portion which both families alike 

 possess, are known as the 'rotulae' or ^falces,' and as they, like 

 the radial elements of the calcareous ring of the Holothurioidea, 

 cover in the junctions of the radial water-vascular trunks to 

 the central water-vascular ring, and as they resemble them still 

 further in not belonging to the perisoraa, but being true internal 

 calcifications, they would appear to be, as Muller taught, homo- 

 logous with them. 



An oesophagus, and sometimes, as in Echinus saxatilis and Spa- 

 tangus, a coecum, is distinguishable at the commencement of the 



