4 ECHINOIDEA. I. 



Sph&r echinus pulcherrimus, Evechinus chloroticus, Echinostrephus molare. - - This instance may be 

 taken as a significant illustration of the generic descriptions. Or should it be necessary also to recall 

 the genera of Cidarids? 



That under such circumstances erroneous determinations have been frequent, is not to be 

 wondered at. I have had occasion to substantiate several (far too many!) cases, and such cases too 

 where the greatest authorities have been responsible for the determination. We ought therefore to be 

 very cautious in using the existing statements with regard to the geographical distribution of 

 these forms. 



The characters that have hitherto chiefly been used for the distinguishing between the genera 

 and species, are the following: the pores, the spines, the tubercles, the mouth-slits, the lining of the 

 buccal membrane with larger or smaller plates, and the calycinal area. All these structures may 

 give excellent characters, and, of course, they are always to be taken into consideration. But most 

 frequently they are so relative, that it is exceedingly difficult or impossible by means of these 

 structures to decide whether a specimen in hand belongs to one species or another. Such is 

 especially the case when the question is of the position of the tubercles; it may be simply 

 irritating to read the descriptions of these in different species that are to be compared, and often the 

 result falls very short of the exertion to get a clear view of the descriptions. To this may be added 

 that the number, size, and position of the tubercles vary very much with age. With regard to the 

 pores, their number and mutual position is no absolutely reliable character either. That in species 

 with many pairs of pores their number increases with age is a well-known fact. The young Strongy- 

 locentrotus drobachiensis has only three pairs of pores (L,oven 250); ^Strongylocentrotus^ lividus has 

 only 3 pairs of pores in the lower ambulacral plates; Echinostrephus has 2 4 pairs of pores, oftenest 

 3 pairs etc 



By these researches the pedicellarise and spicules proved to be of very great systematic 

 importance; they give the most excellent characters we may want. To be sure, this fact is no new 

 discovery. It has long been known that these organs and structures were more or less differently 

 constructed in the different species and genera; much has been written about this fact, and a great 

 many figures have been published. But nevertheless the fact has never been fully utilised. 



The history of the pedicellarise is highly interesting; scarcely many zoological objects will be 

 able to vie with these organs with regard to the number of interpretations. From parasites to 

 embryos, and even to vertebrates, and back again to parasites their history passes, until they are 

 generally acknowledged to be what they really are: organs forming integral parts of the animal, 

 v. Uexkiill has given an excellent account of their history (406), and so there is no reason to give 

 it here again. I shall only here note a few less important treatises, not mentioned by v. U e x k ii 1 1, viz. 

 by Duncan (130), Groom (175), and Stewart (381). A little note by Troschel (Verhandl. d. natur- 

 hist. Vereins d. preuss. Rheinl. u. Westphalen. 1870 p. 137) is also to be mentioned for the sake of 

 completeness; it contains nothing new. 



The histological structure of the pedicellarise has of late years been very carefully studied, 

 especially by Foettinger (155), Hamann (184), Sladen (366), Prouho (327), and v. Uexkiill (406). 

 The most interesting ones in this respect are the globiferous pedicellarice, which have proved to be 



