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Of the species referred to the genus Echinus s. str. two have been stu- 

 died as regards their development, viz. esculentus and acutus. The larvae 

 of these two species are so closely ahke that they are hardly distinguishable. 

 In the first stage they have elongated, clubshaped body rods, but no basket 

 structure; in the second stage they have four epaulets, at the base of the 

 four main arms, and besides a pair of laterally placed epaulets which may 

 form an almost closed ring round the posterior end of the body. There is 

 no posterior transverse rod or posterolateral processes. 



The larva of the nearly related Sterechinus Neumayeri (which Clark 

 regards only as a synonym of Echinus margaritaceus — or Sterechinus 

 Agassizi, as I think its correct name should be) is known in its second 

 stage. It agrees with the two Echinus-lavYSd in all essential features. 



The larvae of these forms accordingly agree very well in their main 

 characters as Ishould be expected, since they are indisputably closely rela- 

 ted. But now those of the other genera ! Here the matter lies quite differ- 

 ently. Within the genus Lytechinus we now know the larvae of the species 

 variegatus, anamesus and pictus in both the first and second stages, and 

 of the species panamensis and verruculatus in the first stage. The young 

 larvae have a short, rounded body, the body rod being short, branching 

 at the end; the recurrent rod is well developed but does not unite with 

 the branch from the body rod so as to form the basket structure, except 

 in the verruculatus-larya.. In the second stage there is a transverse rod, and 

 posterolateral lobes, while the presence of epaulets is not definitely settled. 

 — The larva of Tripneustes esculentus agrees with the Lytechinus-lsLYwae, dif- 

 fering only in the body skeleton of the first stage forming a typical basket 

 structure. The Tripneustes gratilla-lsirya in its first stage has the same 

 structure, while the second stage is unknown. Of Toxopneustes we know 

 with certainty only the young stage of the larvae of T. pileolus and roseus; 

 they agree completely with the Tripneustes-larva in the skeletal structure. 

 Finally the larva of Evechinus chloroticus has in its first stage a more com- 

 plicate body skeleton than that of the Tripneustes-lar\a, though essentially 

 of the basket-type, recalhng that of the Echinometra-larva, and in the 

 second stage (almost certainly) a posterior transverse rod and postero- 

 lateral processes. 



It must be conceded, at least, that the facts from the larvae do not lend 

 support to Clark's view regarding the relationship of these genera. 



Clark's family Strongylocentrotidae encloses the genera Echinoslrephus, 

 Pseudoboletia, Paracentrotus, Loxechinus, Csenocentrotus, Pachycentrotus, 

 Heliocidaris and Strongylocentrotus. Of these the genera Echinostrephus, 

 Csenocentrotus, Pachycentrotus and Heliocidaris according to me belong to 

 the family Echinometridae, Pseudoboletia to the Toxopneustidae, Paracen- 



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