ECHINOIDEA 267 



13. Postero-lateral rods strongly thorny, especially along the posterior 

 edge ; unpaired posterior prolongation not very long, some- 

 times very short (Fig. 3, 4) . . Echinocardium cordatum 

 Postero-lateral rods only slightly thorny ; impaired posterior 

 rods very long (Fig. 191) ... . Spatangus purpureus 



The young stages of the larvae of Psammechinus miliar is, 

 Echinus escidentus, Ech. acutus, and Strong ylocentrotus droha- 

 chiensis are very much alike, but may be distinguished by 

 the shape of the body rods (Fig. I48). The young Paracentrotus 

 lividus larva has fusiform rods like those of Str. drobachiensis, only 

 they appear to be as a rule somewhat more thorny (Fig. 176). 

 In the second stage, the larvae of Psammechinus miliaris and 

 Paracentrotus lividus are easily distinguished from the Echinus 

 and Strongylocentrotus larvae by having only the four epaulettes 



Fig. 148." — Posterior end of body rods in I. larval stage of Psammechinus 

 miliaris (1), Echinus acutus (2), Echinus esculentus (3), and Strongylo- 

 centrotus drobachiensis (4). x ca. 100. 2-3, after Shearer; 1 and 4, 

 after Th. Mortensen. (From Danmark's Fauna.) 



at the base of the postoral and postero-dorsal arms, the latter 

 having also a pair of epaulettes at the posterior end of the body ; 

 these posterior epaulettes are placed laterally, but often widening 

 so as to form a nearly complete ring around the end of the body. 

 But how the larvae within these two groups may be distinguished 

 from each other we cannot see in the present stage of our 

 knowledge ; especially, the larvae of Paracentrotus lividus and 

 Strongylocentrotus drobachiensis need a closer study. 



Regarding the natural relationships of Echinoids no general 

 agreement has yet been reached, several points being still 

 open to discussion. The old division into the two main groups, 

 the Regular {Regularia or Endocyclica) and Irregular Echinoids 

 {Irregularia or Exocyclica), is no natural division, the irregular 

 forms especially not representing a single, natural group. But 

 for practical use the terms " regular " and " irregular " 

 Echinoids are very convenient. 



Not considering some of the old, palaeozoic forms, the 

 Echinoids may be classified in six orders, five of which are 



