6 THE VOYAGE OF H.M.S. CHALLENGER. 



otliev. It seems to me that Loven's attempt to fix the axis of the Echinoidea by means 

 of the position of the plates round the actinostome must of necessity fail. It is at the 

 actiuostome that all the accumulated disturbances during the whole growth of the Sea- 

 urchin concentrate. It is there that constant resorption and crowding is taking place, 

 and while acknowledging the infinite skill with which Loven has attacked 'the subject, 

 many of his conclusions are not of as general application as would at first sight appear. 

 If we examine in the simplest Echinids (the Cidaridse and Salenidaj) the plates as they 

 make their appearance around the apical system, we shall find no such general formula as 

 has been so ingeniously framed for the actinostome. Taking, for instance, the pores at the 

 actinostome according to Loven's formula in a species of Porocidaris we get the usual 

 formula La, Il.rt, IIL6, IV.a, V.&, and we have for the interambulacral system a regular 

 alternation of large and small plates, while at the abactinal system where the plates 

 appear first we get — 



la, ib, 36, 4a, 5a, for the small plates ; and 



lb, 2a, 3a, 46, 5h, „ large 



In a younger specimen, on the other hand, the interambulacral plates alternate regularly, 



and we get — 



\b, 2b, 3b, 4b, 5b, for the small plates, 

 la, 2a, 3a, 4a, 5a, „ large „ 



Their order and formula depeiiding upon the rate of growth of the smaller interambulacral 

 plates, which is evidently a rate quite independent in the different interambulacra, and 

 the formula wiU of course depend upon the particular stage of growth at which the 

 urchin is examined. In two species of Salenia we find the same difiiculty, Loven's 

 formula holding good for the actinostome ; as regards the ambulacra, we have the 

 following formulse difi'erent for the interambulacral plates when we take them as they 

 appear round the apical system — 



16, 26, 3a, 4a, 56, for the smaU plates, 

 la, 2a, 36, 46, 5a, „ large „ 



while in another stage of the same species we have the large and small plates regularly 

 alternating — 



la, 2a, 3a, 4a, 5a, for the small plates; and 



16, 26, 36, 46, 56, „ large 



The same I found to be the case at the apex of Salenia varisjnna. It seems to me that 

 those who have attempted to define the antero-posterior axis of the Echinids have failed 

 from their not taking into account the embryological data? These, it is true, are scanty, 

 yet they are sufiicient to settle the question of the all importance of the madreporic body. 



