Prof. S. Loven on the Structure of the Echinoidea. 291 



by the genital and eye-plates perpendicularly above the mouth, 

 and the corona, which is nowhere in contact with it, developes 

 in like manner its ambulacra and interradia. The apparently 

 regularly radiate form is originally disturbed by the madre- 

 poric apparatus, which perforates with its strainer one of the 

 genital plates, all of which subsequently, during the early 

 growth of the animal, become perforated by the efferent ducts 

 of the genital glands. That even here the genital plate which 

 contains the madreporic plate is the right anterior one, and 

 that the ideal longitudinal axis of the body passes through 

 the unpaired ambulacrum, as thereby indicated, is confirmed 

 by the fact, that only by such a division between right and 

 left does the same formula prevail for the plates of the peri- 

 stomial margin in the regular as in the irregular forms. Tliis 

 is most distinctly observed in very young individuals, in which 

 the primary plates may still be distinguished. 



If a young Toxojjneustes drohachensis of from 3 to 6 millims. 

 diameter be held with the mouth upwards, and the unpaired 

 ambulacrum, determined as above, forwards, and the peri- 

 stomial plates be gone through in the same sequence as was 

 adopted in the examination of the irregular Echinoidea, we 

 find not only that all the peristomial plates are composite 

 (and may therefore be denominated large plates), but also that 

 I. a, II. a. III. hj IV. a, V. h are all ternary ; that is to say, 

 every one of them consists of three still distinguishable primary 

 plates ; Avhilst I. hj II. h^ III. a, IV. h^ V. a are binary, formed 

 of two primary plates. Consequently here also the peri- 

 stomial plates of series I. a-V. h are larger than those of series 

 I. h-Y. a, and likewise bear more pores. In both series, the first 

 primary plate has two pores, a complete double pore and one 

 which is formed only by a notch in the very margin ; and it 

 may be supposed that this primary plate is a combination of 

 two plates which were distinct in a still younger stage, the 

 earliest formed of which, like all others, had a complete double 

 pore, which afterwards, during growth, shifted to the margin 

 and became reduced, its upper passage being closed and its 

 lower one partly removed and thus converted into merely a 

 notch of greater or less depth (see Plate XIV. figs. 1 & 2-8). 



The primary ambulacral plates in the Latistelte are in part 

 entire, i. e. such as occupy the whole space between the intcr- 

 radium and the median suture of the ambulacrum, and in part 

 halved, or such as extend from the interradium to about "the 

 middle of the entire ones, and terminate there in a more or less 

 distinct point. The larger peristomial plates of the ambulacral 

 series I. a-V. h generally consist, in very young individuals, of an 

 entire adoral, a half intermediate, and an entire aboral primary 



