REPOKT ON THE ECHINOIDEA, 7 



hoih physiologically and morphologically ; and further, they have failed from not sufB- 

 ciently taking into account the dorso-central system of Clypeastroids, and in attempting 

 to pass at once from the Goniocidaridse (the regular Echiuids) to the Spatangoids 

 (Petalosticha) have left out an important term of comparison. 



From embryological data ^ the madreporic body indicates in Echinids, Starfishes and 

 Ophiurans the line along which the suture of the open spiral of the young Echinoderm 

 has taken place. It is the only body in the regular Echinoidea which can denote any 

 axis, and from the mode of development of the interambulacral system, after the 

 ambulacral, merely indicates that the two ambulacra adjoining it are developed at the 

 opposite ends of the open spiral once forniing the young Echinoderm ; so that any starting- 

 point we wish to take, in making out formulae for the arrangement of the plates, ought to 

 be chosen with reference to the position of the madreporic tubercle, and should be either 

 the one to the right or to the left of it, that is, either of the ambulacra which Lov^n has 

 numbered III. and II. They mean something, and have a definite value, which the others 

 have not ; and the fact that the right anterior interambulacrum frequently contains the 

 madreporic body in Spatangoids is no proof that the interambulacral area of regular 

 Echinids which contains it is the right anterior interambulacmm, as has been supposed 

 by the majority of writers on the Echinoidea. 



Nothing in the position of the anus can help us to determine in the Desmosticha which 

 is the odd interambulacrum except the tendency we see in some genera of the anal 

 system to approach that interambulacrum ; since while within the genital ring nothing in 

 its position can guide us to any axis corresponding to that of other sub-orders where 

 additional structural features leave us no doubt of its position. I have shown in the 

 Kevision of the Echini that the general trend of the alimentary canal and its windings 

 are of no assistance in this matter, and that the position of the anus in different 

 genera of Echinometradse shows that, in the regular Echinoidea at any rate, it cannot be 

 used to determine any axis ; while, on the contrary, the position of the anus in the 

 Clypeastroids and Petalosticha, and the frequent specialisation of one of the ambulacra 

 in the latter, gives us a ready clue to fix the axis of these groups. 



The youngest Echinids I have examined, immediately on the resorption of the Pluteus 

 {Mem. Am. Acad., 1864), show plainly why we should not have the relations between the 

 different ambulacral and interambulacral plates discovered by Loven limited to a single 

 one of the zones. The first trace of the ambulacral system in each ambulacrum 

 consists of five loops becoming subsequently five tentacles, which are absolutely 

 similar. These large embryonic tentacles are not, as Loven supposes, temporary, 

 as can be readily seen on examining the figures of the paper referred to above, which 

 shows the gradual increase in number of the original tentacles, the mode of formation of 



' See A. Agassiz, Embryology of Echinoderms, Mein. Am. Acad., 1864 ; A. Agassiz, Embryology of the Starfish, 

 '1864. . 



