ECHINOIDEA. I. 



95 



not the same as what Rotlijsletz has made of it. At all exents Hesse is right, when he savs that 

 the cancellate structure is oiily compliciertere Wachsthumserscheinmigen an Stacheln seines zweiten 



Baujjlanes, so dass die Stacheln ein nnd derselben Species, z. B. von Strongylocentrotus alhus Ag 



je nach dem Stadinm ihrer Verdickung tlieils zn den Radiaten, theils zn den Cancellaten zii rechnen 

 sein wiirdens (op. cit. p. 192). — To jndge by what has hitherto been bronght to light, we ma\- scarcely 

 expect to find featnres of any greater systematic importance in the structure of the spines witli regard 

 to the forms treated of here. 



The gills will scarcely present peculiarities that may be used as systematic characters of 

 greater importance. They generalh' contain some irregular spicules and fenestrated piates, which are 

 in the lower part rather large and pass evenly into the piates of the buccal membrane; towards the 

 ends of the branches they become smaller and more irregular, at last only branched calcareous needles. 

 Common bihamate sjjicules are most frequently found together with these, sometimes in very great 

 numbers [Pscudobolcfia). Ilcterocciitrohis and Colobocciitrotiis are distinguished b}' having j^edicellariæ 

 on the gills (placed on the larger fenestrated piates). In Stomopncustcs only small three-radiate spicules 

 are found in the gills (PI. XVII. Fig. 13). — The sphæridiæ are very similar; tlieir shape, number, or 

 position can in no way be used as distinguishing characters between species, genera, or greater groups 

 within this division of the Echinids. 



The buccal membrane may be covered with piates, or naked, and this feature has played no 

 small part in the classification, and will also persistently be of imjDortance. It is, however, to be 

 observed that it cannot always be seen directly whether piates are found in the buccal membrane or 

 not. Often it looks quite smooth and naked — as for instance in EcJuims acufiis — but if a piece of 

 it is cleared in potash or Canada balsam, it is seen to be quite full of larger or smaller, simple fenes- 

 trated piates; only when these piates carry pedicellariæ the)- become more complicate, and may then 

 be seen on the dried skin. Thus a microscopic examination is necessary in order to ascertain whether 

 piates are found in the buccal membrane or not. j\Iost frequently among the fenestrated piates more 

 or fewer spicules of the common bihamate form are found. The part inside of the buccal piates gene- 

 rally contains numerous smaller fenestrated piates, arranged more or less radially; these piates are 

 upon the whole more simph constructed than those outside the buccal piates. In several species the 

 buccal membrane is almost or quite naked (with the exception of the buccal piates), for instance 

 Echinus magellaniciis^ alboa'ncfus^ Robillardi. In some species small spines are found on the buccal 

 piates (for instance Ech. csculcntus\ and in Psnidobolcfia, Hctcroccutrotiis^ and Colobocentrotus spines are 

 even found in the piates of the buccal membrane outside the buccal piates. 



The inner anatomicai structures are especially little known in the different genera, with the 

 exception of the dental apparatus and auriculæ. These, however, show a so similar structure, that 

 important differences that might be of systematic significance, are scarcely to be found, and as to the 

 other anatomicai features, it is still more improbable that here should be found differences of any 

 importance — apart from the faet that it would be ver}- unpractical, if the inner anatomy was to be 

 much used in the classification. Thus we have ouly left spicules and pedicellariæ — but here we also 

 find what we want. 



