Io6 ANATOMY AND DEVELOPMENT OF THE ECHINODERMS 



in the latter inside it ; but this apparent difference arises only from a 

 difference in the mode of development of the ambulacral plates. 



The ambulacral plates in the Asteridae, between which the canals 

 lead from the ampulla; to the feet, are homologous with the ambulacral 

 plates in the Echinida; tlirough which they pass. 



But in the Asteridse the ambulacral plates develope internal pro- 

 cesses which unite above, or internal to, the water-vascular canals and 

 nerves, while in the Echinidse the ambulacral plates unite below or 

 external to the water-vascular canals and nerves. 



In the EchinidEe, the only parts that represent the internal pro- 

 cesses of the Asteridae are the " auricula; " — arched processes which 

 give attachment to the suspensor muscles of the lantern, and under 

 which the vessels and nerves pass. 



In the Ophiuridas both internal and external processes of the am- 

 bulacral plates exist, and the vessels and nerves are contained in a 

 complete bony canal. 



In the Holothuriadae the arrangement of parts is as in the Echinida;. 

 The ring, composed of ten to fifteen bony pieces, encircling the oeso- 

 phagus, is not homologous with any part of the skeleton of the 

 Echinidae, but with the lantern or masticatory apparatus. 



Five of these pieces are always either notched (as in the Holo- 

 thuriadse) or pierced (as in the Synaptae) for the passage of the water- 

 vessels and nerves, and these pieces correspond homologically with 

 an equal number of calcareous pieces of the lantern of the Echinida 

 (fakes of Valentin) which cover in the terminations of the radial water- 

 canals in the circular canal. 



Ever}' Echinoderm commences its existence as an oval ciliated 

 body like an infusory animalcule, without organs or distinction of 

 parts. 



In some genera, such as Asteracanthion and Echinaster, it appears 

 from the observations of Sars, Agassiz and Desor, that such a germ 

 as this developes at one part one, three, or four short processes or 

 peduncles, by which it is enabled to adhere to other bodies ; among 

 these Prof Mtiller thinks he has discovered an aperture. The re- 

 mainder of the germ gradually enlarges and assumes the form of a 

 starfish. The feet appear on its under side whence the peduncle or 

 peduncles proceed. The latter become smaller, and eventually appear 

 as mere processes on one side of the mouth of the young starfish, finally 



V 



anishing altogether. 



t>"- 



Xow in these larvae, their inner structure and the mode in which 

 the disc of the starfish is developed do not appear to have been clearly 



