ANNULOIDA: ECHINODEKMATA. 131 



The order Asteroidea has been divided by Dr. Gray as fol- 

 lows : 



ORDER ASTEROIDEA. 



Section a. Ambulacra with four rows of feet. 



Family 1. Asteriadce, Dorsal wart simple. 

 Section b. Ambulacra with two rows of feet. 



Family 2. Astropectinidce. Back flattish, netted with numerous tu- 

 bercles, crowned with radiating spines at the tip, called ' paxillse.' 

 Family 3. Pentacerotidce. Body supported by roundish or elongated 

 pieces, covered with a smooth or granular skin, pierced with minute 

 pores between the tubercles. 



Family 4. Aster inida. Body discoidal or pyramidal ; sharp-edged ; 

 skeleton formed of flattish, imbricate plates; dorsal wart single, 

 rarely double. 



ORDER OPHIUROIDEA. This order comprises the small but 

 familiar group of the 'Brittle-stars ' and ' Sand-stars,' often con- 

 sidered as belonging to the Asteroidea, to which they are nearly 

 allied. The body in the Ophiuroidea (fig. 35) is discoidal, and is 

 either naked, or is covered with granules, spines, or scales. 

 Prom the body which contains all the viscera proceed long, 

 slender arms, which may be simple or branched, but which do 

 not contain any prolongations from the stomach, nor have their 

 under surface excavated into ambulacral grooves. The arms, 

 in fact, are not simple prolongations of the body, as in the As- 

 teroidea, but are special appendages, superadded for locomotive 

 and prehensile purposes. Each arm is enclosed by four rows of 

 calcareous plates, one on the dorsal surface, one on the ventral 

 surface, and two lateral. In the centre of each arm is a chain 

 of quadrate ossicles, forming a central axis, and between this 

 axis and the row of ventral plates is placed the ambulacral 

 vessel. The mouth is situated in the centre of the inferior 

 surface of the body, is provided with a masticatory apparatus, 

 and is surrounded by tentacles. It opens directly into a 

 ciliated stomach, which is not continued into an intestine, 

 the mouth serving as an anal aperture. The reproductive 

 organs are situated near the bases of the arms, and open 

 by orifices on the ventral surface of the body or in the inter- 

 brachial areas. 



It is very questionable whether the ambulacral system in 

 the adult Ophiuroidea communicates with the exterior; its place 

 as a locomotive apparatus being taken by the arms. 



The larva of the Ophiuroidea is pluteiform and is furnished 

 with a continuous eiidoskeleton ; and in some, as in Oplno- 

 lepis squamata, the echinoderm-body appears within the larva, 

 when the latter has attained but a very imperfect degree of de- 

 velopment. 



K 2 



