94 



ECHINOIDEA. I. 



even- (ambulacral) plate has its primary tubercle well developed*. He has not, however, used this 

 feature as a systematic character. On the other hand Diiben & Koren 1 ) and G. O. Sars 2 ) have 

 carefully noted this fact in their descriptions, and Koehler (233. a) has recently given prominence to 

 this feature in his description of Sterechinus antarcticus. 



The apical area, no doubt, shows some difference: sometimes all the ocular plates are shut off 

 from the periproct, sometimes one or more reach to it. That no greater importance can be attached 

 to this feature is a sure fact, which may be seen with especial clearness from a case as that of Ster- 

 echinus antarctiais (=Ech. inargaritaccus), where in young individuals all the ocular plates are shut 

 off from the periproct, while in the adult they reach, all of them, to it (Koehler, 233. a). 



The structure of the spines does not seem to yield very good systematic characters. Mackin- 

 tosh (265) has given numerous excellent figures of transverse sections of spines from a great number 

 of species. But I do not think that he has found so great and reliable differences in this feature, that 

 it can be used as a criterion of a nearer or farther relation between the separate forms. Especially 

 I think that a greater variation in the structure of the spines of the same species may be found, than 

 is to be seen from the work quoted. Also the secondary spines of the different species ma}- deserve 

 a nearer examination. Hesse (195.3) has recently made thorough studies of the structure of Echinid- 

 spines, especially the fossil ones. He arrives at the result, cdass fast jede der einzeluen Familien der 

 Echinoideen ihreu eigeueu mikrostrukturellen Stacheltypus besitzt, und dass die histologischen Ver- 

 haltnisse der Stacheln em wichtiges systematisches Kennzeichen fiir die Familien und in gewissen 

 Ziigen von secundarer Werthigkeit oft sogar fiir die Gattungen, ja fiir einzelue Arten der Seeigel 

 liefern (p. 204). He establishes 6 types: Cidaris, Echinus, Diadema, Ciypeaster, Scufcilidir, and S pa- 

 tan gus, and if we take the families to be of a corresponding extent, the spines may be seen to yield 

 family -characters. The type of Echinus comprises both Temuopleurids, Echinometrids, and Echinids 

 s. str. He divides them into two parts, a) with the radial septa not perforated, b) with the radial septa 

 perforated. To the first division belongs among others Toxopueustes pilcolus, to the second Hipponoc 

 esculenta — two forms that are no doubt very nearly related. Such things prove how little value is 

 to be ascribed to this character. Upon the whole it must be said that the structures mentioned by 

 Hesse will scarcely be of any great importance with regard to the recent Echinids; with regard to 

 the fossil ones, on the other hand, they will, no doubt, be of some importance, as we may always from 

 the structure get some instruction with regard to the correct referring of the animal or the single 

 spine, even if it will only in rare cases be possible to get at the genus or the species. — Rothpletz 

 (346. p. 289) says of Radioli caucellati (corresponding to the <polycyclic acanthospheuote spines of 

 Mackintosh): <Nach Agassiz ware dieser letzte Typus auf die Familie der Echiuomcfradir beschrankt, 

 wahrend der zweite Typus (Rad. radiati) alien iibrigeu Familien mit Ausnahme der Cidarideu und 

 Saleniden zukame . As far as I can see Agassiz has said no such thing; in Rev. of Echini (p. 654) 

 he says: < In the EchinometradcR we find the concentric rings most distinctly developed-; but that is 



') Skandinaviens Echinodenner. Vet. Akad. Hand! 1S44. 



2 ) Nye Echinodenner fra den norske Kyst. Vidensk. Selsk. ForhandL 1S71. p. 23 (in the description of Ech.depressus 

 |= noi-vegicus\). 



