760 HOMOLOGIES AND AFFINITIES OF ECHINI. 



We must also be cautious, in tracing the homologies between the ambula- 

 cra! plates of Starfishes and Echini, to remember that the homology extends 

 only to the passage of the ambulacral suckers, and that the position of the 

 ambulacral tube is totally different in the two suborders, as has already been 

 insisted upon by Miiller ; the ambulacral tube being placed behind the 

 plates (in the interior of the test) in Echini, while in starfishes the ambu- 

 lacral tube is situated between these plates in a channel formed In their 

 sides. We can only homologize the auricles of the Echini to the continua- 

 tion of the ambulacral plates of Starfishes in those genera in which the 

 auricles are formed by the prolongation of the ambulacral, and not of the 

 interambulacral processes. As far as the jaws of Echini are concerned, they 

 do not belong, as I have already shown, to the test, they are not a pari of the 

 auricles, and hence are in no way to be compared to the actinal parts which 

 have received the name of teeth in Starfishes and Ophiurans. and which prob- 

 ably correspond to the auricular processes. 



I have nothing to add regarding the plan of development of Echinoderms 

 in the different suborders. In addition to the memoirs of Miiller but little 

 has been added to our knowledge of the history of the order since the pub- 

 lication of my Embryology of the Starfish, beyond a Memoir of Metschnikoff. 

 In this he has taken much the same view of the development of the Echino- 

 derms which 1 first advocated in opposition to the views of Miiller. The 

 development of the Echinoderm, as 1 have shown, takes place in all the sub- 

 orders upon the so-called water-system of the pluteus. — the pluteiis. emi- 

 nently bilateral, showing no trace whatever of the radiate struct inc. — and 

 it is only when the Echinoderm has grown to lead an independent life that 

 the radiate structure so characteristic of the adult is developed. The water- 

 system is the only organ in the pluteus showing any approach to a radiating 

 disposition; but this, alter all, is really only a fan-shaped arrangement, for 

 the rudimentary Echinoderm. till the last moments of the existence of the 

 pluteus, can only be said to exist developed (like a surface in geometry) on 

 the surface of the water system, while all the organs of the pluteus. the arms. 

 the vibratile cord, the epaulettes, the calcareous rods, or wheels, and other 

 limestone deposits, are perfectly symmetrical, placed on both sides of the 

 longitudinal axis, which also divides the digestive cavity and oesophagus into 

 equal halves. 



As far as the affinities of the different suborders are concerned, we obtain 

 remarkable results if we consider the pluteus alone. Some of the Holothu- 



