Characteristics of Aster oidea. cliii 



difietl feet homologous with the Holothurian tentacles on the peristomial 

 ai'ea, and the peculiarities of the digestive tract. The ordinary represen- 

 tatives of these two classes resemble each other in the small size of their 

 anti-ambulacral area, and in the tendency they have to assume bilaterally 

 symmetrical forms. Their developmental history, however, is very 

 different. 



Class, Asteroidea. 



Echinodermata, with flat, star-shaped, or simply pentagonal, bodies, 

 with a well-developed and functionally locomotor water-vascular 

 system, the ambulaeral surface in relation with which is developed 

 commensurately with the anti-ambulacral, centrally or sub-centrally 

 in which the anus, when present, opens. 



The Asteroidea differ from the other Echinodermata in havino" a 

 well-developed ' internal skeleton/ externally to which their nerve - 

 cords and radial ambulaeral vessels lie. This system is represented 

 Tudimentarily by the auriculae of EcJiinidae, and the similar but 

 more developed internal structures of Clypeaster ; but it is wholly 

 wanting" in the Crinoidea. 



The Asteroidea are divided into two sub-classes, the Asferiae and 

 the OpMuridae ; in the former of which the arms, firstly, are pro- 

 longations of the central disc ; and, secondly, contain within them 

 prolongations of the digestive tract, and portions, or the whole of 

 the reproductive organs ; and thirdly, are furrowed on their medio- 

 ventral surface for the reception and protrusion of the ambulaeral 

 feet; whilst in the latter, the arms are differentiated from the 

 central disc, not only by their mode of taking origin from it, 

 but also by not containing any portions of the viscera of or- 

 ganic life, and by not having any medic-ventral ambulaeral 

 furrows, but in place of them, ordinarily, a row of dermal 

 scales, on either side of which the feet are protruded. The verte- 

 bral ossicles of the OpMuridae resemble those of the Asteriae in 

 being composed of two symmetrical halves ; but these two halves 

 are articulated together immovably, except in the pair next the 

 mouth, whilst the entire series of mesial ambulaeral ossicles is in 

 Asteriae composed of mesially and movably articulated bilateral 

 halves, for the adduction and divarication of which special muscles 

 are developed. The OpMuridae have not the internal prolongations 

 of the branches of the radial vessel to the feet, which arc known in 

 Asteriae and the other higher Echinodermata as ampullae ; but 



