132 



ECHINOIDEA. I. 



The same form of globiferons pedicellarice is found in Strongylocenfrotus^ Aiitliocidaris^ and 

 Parasalenia. The two former are distinguished by the globiferous pedicellarise having a well devel- 

 oped neck, provided with circular and longitudinal muscles — an otherwise unknown feature. These 

 three genera are likely to be rather nearly related; their spicules, however, show that the relation 

 is not \ery close. In Stroiigylocentrohis the spicules are a little branched in the ends, but otherwise 

 the original form is bihamate; in some species onh- (?) common bihamate spicules are found. In 

 Aiifhocidaris the spicules are biacerate, pointed in both ends and with a branch in the middle. A 

 somewhat similar form of spicules is found in Parasaloiia; but in this genus the globiferous pedicel- 

 larise have no neck. Thus this latter seems to form a special group; its obliquity and the peculiar 

 anal plates indicate also that it must be interpreted as an aberrant form, of which the nearest, 

 although not very near, relations are: Anthocidaris and Slrongylocr/ifrofns. In the genera Heliocidaris^ 

 Echinostrrplius^ Toxocidaris, Echinometra., Heteroccnirottis^ and Coloboccntrotiis there is a strong, unpaired 

 lateral tooth on the globiferous pedicellarise, and they have all simple bihamate spicules. Hcliocidaris 

 occupies a somewhat isolated position; its globiferous pedicellarise are not so much developed as those 

 of the other genera, it reminds to a rather high degree of Stcrcchiinis Nriunoyrn\ but especiall\- of 

 Psmdechinus albocincfiis\ several things favour the belief that Psendcchiints is really a transitional 

 form between Stcrccliinus and I Teliocidaris^ and the latter leads on again to Toxocidaris^ Ecliiiioiiirtra 

 etc. Thus we have here a very fine series of development where, together with the peculiar develop- 

 ment of the globiferous pedicellarise, a ;narked tendency to obliquit\- is seen, reaching the climax in 

 the genera liefcrocnitrotus and Coloboccntrotiis. There seems to be no occasion to separate these two 

 genera as a .special group on account of their longitudinal axis not being placed in the same direc- 

 tion as in Echinomctra^ because their pedicellarise and spicules are exactly agreeing with those of 

 Echiiioinctra. It is constantly seen that spicules and pedicellariae are the most important systematic 

 characters, so that there is no reason for suddenly following a new principle here. The genera Pscud- 

 cc/ihnis, Heliocidaris, and Echinostrcphus must then be interpreted as more or less primitive oligopore 

 Echinometrids. 



StomopHciistcs occupies a quite isolated position; its globiferous pedicellarise and spicules are 

 so peculiar and so different from what is found in the other forms mentioned here, that there can be 

 no question of classing it with an\- of them; it forms a special group. 



The relation between these forms may most easily be surveyed in the following diagram. For 

 safety's sake I shall expressly remark, however, that I do not mean it to be regarded as a phyloge- 

 netic one. I will in no way maintain that our Pareclihius is the ancestral form of Echinus etc., but 

 only express my opinion that it shows the simj^lest structure of thejorgans most important with regard 

 to classification. We may in the recent forms scarcely find more than an indication of the way the 

 development seems to have taken. Now there is unfortunately onl\- a small chance of finding these 

 fine structures in the fossil forms, so we shall hardly get so far as to be able with certainty to point 

 out the ancestral forms. Otherwise this surve\' of the relations of the forms shows clearly that here is 

 everywhere a tendency to increase the number of tube feet, a development from oligopore to polypore 

 f(jrms. The most original feature, no doubt, is that all the ambulacral plates are well developed with 

 primary spine and three tube feet; then the ])rimary spines disappear from every other ambulacral 



