4 Prof. Miiller on the Anatomy and Development 



The position of the water-vascular canals and of the nervous 

 cords is apparently different in the Asteridse from what it is in 

 the Echinidse, inasmuch as in the former these organs are out- 

 side the bony skeleton, in the latter inside it ; but this apparent 

 difference arises only from a difference in the mode of develop- 

 ment of the ambulacral plates. 



The ambulacral plates in the Asteridse, between which the ca- 

 nals lead from the ampullae to the feet, are homologous with the 

 ambulacral plates in the Echinidse through which they pass. 



But in the Asteridse the ambulacral plates develope internal 

 processes 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 Echinidse, the only parts that represent the internal 

 processes of the Asteridse are the " auriculse " — arched processes 

 which give attachment to the suspensor muscles of the lantern, 

 and under which the vessels and nerves pass. 



In the Ophiuridse both internal and external processes of the 

 ambulacral plates exist, and the vessels and nerves are contained 

 in a complete bony canal. 



In the Holothuriadse the arrangement of parts is as in the 

 Echinidse. The ring, composed of ten to fifteen bony pieces, 

 encircling the oesophagus, is not homologous with any part of 

 the skeleton of the Echinidse, but with the lantern or masticatory 

 apparatus. 



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

 Holothuriadse) or pierced (as in the Synaptse) for the passage of 

 the water-vessels and nerves, and these pieces correspond homo- 

 logically with an equal number of calcareous pieces of the lantern 

 of the Echinidse (falces of Valentin) which cover in the termina- 

 tions of the radial water-canals in the circular canal. 



Every Echinoderm commences its existence as an oval ciliated 

 body like an infusory animalcule, without organs or distinction 

 of parts. 



In some genera, such as Aster acanthion 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. Miiller thinks he has discovered an 

 aperture. The remainder 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 vanishing altogether. 



Now in these larvse, their inner structure and the mode in 



