46 Hjalmar Théel, 



their position. Thus, in the year 1852 *) he explains that they ought 

 not to be referred to the Clypeastroids i. e. Echinocyamus. Here he 

 says: »Es bleibt daher noch ein Problem, die Art oder die Arten für 

 die Seeigellarven mit gegitterten Kalkstäben zu bestimmen». However, 

 in the year 1855 ^) he expresses his decided opinion that these larvfe 

 must probably belong to Echinocyamus. 



As I have mentioned above, our present knowledge of the deve- 

 lopment of the Clypeastroids is very defective and, if we except the papers 

 of I. Müller, is confined to an account of the development of Echi- 

 narachnius^) and a very fragmentary sketch on Mellita*). It is of spe- 

 cial interest to note that the Pluteus of Echinarachnius bears such a 

 resemblance to that of Echinocyamus, that it seems almost impossible to 

 distinguish the one from the other. Judging from the description given 

 by Fewkes the only difference may be »the existence of large pigment 

 spots near the distal ends» of the larval arms in Echinarachnius, which 

 are not present in Echinocyamus. 



Considering that, in spite of the rich literature, only comparati- 

 vely few pluteus-larvae are known so well as to make it possible to 

 refer them to fixed species of sea-urchins, it is of course not possible 

 to draw comparisons between them in order to gain any general results 

 of value. 



It has hitherto been a common idea that the Pluteus of the regular 

 Echinoids is provided with cilialed epaulettes which are absent in the Cly- 

 peastroids and Spatangoids. However, so far as is known, this appears to 

 be accurate only with regard to a limited number of the sub-order Echi- 

 nida3. On the other hand the Spatangoids are supposed to be distin- 

 guished by possessing an unpaired rod at the lower end. Though this 

 seems to be a rule in the irregular edentated Echinoids, it does not 

 exclude the possibility of such a rod being found in other larvEe. Thus, 

 for instance, Prouho^) speaks of a terminal rod in a larva of Dorocidaris 



1) Ueber die Larven und die Metamorphose der Echinodermen. ■ — Abh. 4. 

 1852. Sep. p. 27—29. pi. VHI. fig. 9. 



2) Ueber die Gattungen der Seeigellarven. — Abh 7. 1855. Sep. p. 22—31. 



pi. vm. 



3) PreHniinary Observations on the Development of Ophiopholis and Echi- 

 narachnius. 1886 



4) Preliminary Notes on the Echinoderms of Beaufort. 1885. 



5) Recherches sur le Dorocidaris papillata et quelques autres Echinides de la 

 Méditerranée. 1887. p. 349—350. pi. XXA^. fig. 9. 



