294 



.ECHINODERMAIA. 



no diverticula of the alimentary canal. The movements of the arms 

 are principally in the horizontal plane, and in many cases permit of 

 a creeping locomotion amongst marine plants. The anibulacral 

 groove is always covered by special dermal plates, and the anibulacral 

 feet project laterally between the spicules and plates on the upper 

 surface (fig. 238). In a few cases the arms are branched, and can be 

 rolled up in the direction of the mouth. In such cases the ventral 

 groove is closed by a soft skin (Astrophyton). The anus is always 

 wanting, as are jtedicellarias. The generative products pass into 

 genital pouches (bursse), and from these directly to the exterior 



through inter-radial paired 

 slits. The madreporic plate 

 lies upon the ventral sur- 

 face in one of the buccal 

 plates. Some few Ophiu- 

 rids are viviparous, e.g., 

 Amphiura squamata; these 

 do not undergo metamor- 

 phosis. Most pass through 

 the Pluteus larval stage, 

 e.g.. Ophiogtypha Lyra., 

 (Ophiolepis) ciliata with 

 larval stage Pluteus 

 paradoxus. 



FIG. 238.-Op1tiothrij:fragi!i8. The ends of the rays 

 have been removed. OS, Slits of the genital 

 pouches ; K, masticatory ossicles. 



Faru. Ophiuridae. With 

 simple unbranched arms, and 

 with ventral plates to the 

 ambulacral groove. They are divided into special genera according to 

 the pec-uliar character of the dermal covering and of the buccal armature. 

 Opltiotliric Mull. Tr. The back is provided with granules, hairs, or spicules. 

 The lateral plates of the arms bear spicules. Opk. fray His 0. Fr. M tiller. 

 Ophiura Lam. [OpTlioderma). Two pairs of genital slits in each intcrbrachial 

 space. 0. longicauda Link., Ophiolepis Liitk., Amphiura Forb. 



Fam. Euryalidae. Mostly with branched arms which can be curved towards 

 the mouth and are without plates ; the ventral groove closed with soft skill. 

 Astroplujton rcrritcuxum- Lam.. Indian Ocean. -1. arlorcscens Eond., Mediter- 

 ranean. Asteronya: Lovciti Mull. Tr. 



CLASS III. ECHINOIDEA,* SEA-URCHINS. 



Spherical, heart-shaped, or disc-shaped Echinoderms icith immovable, 

 skeleton composed of calcareous plates. The skeleton encloses the body 



* Besides the works of J. Th. Klein, compare E. Desor, " Synopsis des 

 Echinides fossiles," 1854 to 1858. S. Lovcn, "Etudes sur les Echinoidees," 

 Stockholm 1874. Al. Agassiz, "Revision of the Echini," Cambridge, 1872- 

 1874. 



