Introduction to A nimal Morphology. 131 



(5, Ag}} ; arms long, close, four (five, Ag.) pair ; these, like 

 thelast, are West Indian. 3. Ophiocrinus — with five undivided 

 arms, sixteen ventral cirrhi, and no other ossicula. 4. Rhizo- 

 crinus — with a long, jointed column, and long rounded joints ; 

 calyx variable in its number of pieces ; anus circular, midway 

 between the mouth and the margin of the disc (North Atlantic). 



Comatulidae — feather stars ; free when mature ; stalked 

 when young; body with immovable dorsal cirrhi of 2-180 

 joints, representing those of the stem in Pentacrinus ; 10-40 

 pinnulated arms ; anus ventral. They quickly pass through 

 the early stages of development, when they appear as ovate 

 bodies with four transverse rows of cilia, a keyhole- shaped 

 mouth and an anus. The stalked condition has been named 

 Phytocrinus. The genera are Antedon, the common feather- 

 star, from which possibly Alecto may be distinct ; and Pha- 

 nogenia, with a simple stelliform centro-dorsal joint, and basal 

 plates hidden, internal (from Malacca). Allied, are the fossils 

 Hertha, Pterocoma, &c. 



4. Ophiuroidea — brittle stars, w'ith rounded or pentagonal 

 flattened discs, and elongated arms differentiated from the 

 body, and containing no viscera. The ambulacra! pedicelli 

 have no ampullae nor sucking discs, and feet are developed 

 medio-ventrally. Pedicellarice are wanting. There is a cal- 

 cified peristome of oral plates, grooved for the nerve ring, 

 and armed with palae angulares, or spurious teeth, based on 

 five small plates {tori angular es^^), one at each of the five 

 angles of the mouth. The disc may be naked (Ophiomyxa, 

 Ophioscolex) ; or covered with fine, hard, equal granules 

 (Ophiocoma, Ophiarachna) ; or with shields (Ophiomastix) ; 

 or granules and scales (Ophiothrix) ; or many large, symme- 

 trical shields (Ophiolepis). The arms are special locomotive 

 organs, each enclosed in four rows of plates, dorsal, ventral, 

 and two lateral, within which is an axis of jointed, squarish, 

 dermal, calcareous plates, separated from the ventral surface 

 shields by the ambulacral vessel, and above the axis is the 

 nerve cord. Each plate consists of two ankylosed halves, 

 and there is a branch of the nerve and water vessel for each 

 joint of the axis. The first pair of axial segments is large, 



* Possibly homologous with the teeth of Echini (?) 

 K 2 



