134 Introduction to A n imal Morphology. 



back of the arm insertion (which notches the disc) has a row 

 of papilla, either medially interrupted (Ophiura), or not 

 (Ophiocten). 



Order 5, Brisingoidea — passage forms, with the disc of 

 Asteriadge and the arms of Ophiuridse ; closely bristled, with 

 eleven long cylindrical arms, separate from the disc, but con- 

 taining short, single coeca from the stomach ; anus sub- 

 central ; pedicellarias present ; madreporiform plate marginal, 

 dorsal ; genital glands limited to the middle of the arms, 

 opening dorsally by two sets of pores. The open ambulacral 

 grooves, which do not extend to the mouth, have two series 

 of feet. The one species, Brisinga endecacnemos {AsbJornsc7i), 

 is found on the Coast of Norway, as well as in the Southern 

 Ocean.* 



Order 6. Asteroidea — Starfishes having the arms conti- 

 nuous with the disc, being, in fact, its prolonged angles. 

 The dorsal ant-ambulacral surface is covered by a coloured, 

 leathery membrane, bearing bristles (paxillse) and compound 

 spines, sometimes moved by muscles. The madreporiform 

 plate is conspicuous, dorsal ; sometimes there are several, 

 and as many stone-canals (five in Echinaster). In Pteraster, 

 it lies with the anus in the floor of a sub-cutaneous dorsal 

 pouch. The lower surface of each arm is medially grooved 

 for the ambulacral feet, which are at least in double rows, and 

 often bordered by warty papillae {papilla: sulcoriwi). Under 

 the leathery membrane is a dermal skeleton of many vertchnc 

 or segments, each consisting of several pieces {adambulacml, 

 intermediate, ventro-marginal, uniting, and dorso-margi7tal). In 

 Astropecten, there are over 12,000 pieces ; in Uraster, 1 1,000. 

 Each arm possesses a muscular system, consisting of: — 

 I St. Transverse fibres above and below each segment, joining 

 together the two lateral halves, which are movable on each 

 other ; not ankylosed, as in Ophiuridse. 2nd. Inter-vertebral 

 longitudinal muscles, smaller than the corresponding set in 

 Ophiuridae. 3rd. Inter-ambulacral, longitudinal. 4th. Trans- 

 verse bands from the ad-orally expanded sides of the vertebras 

 to the floor of the ambulacral groove, which they can narrow. 



Scattered over the body, or about the mouth, are prehen- 



* Possibly the fossils Protaster, Palaeodiscus, &c., may belong here. 



