2IO DISCOVERY REPORTS 



particularly the oral primaries are quite different from those of the normal form, not 

 coarsely serrate as in the latter, but finely thorny like the other primaries and much 

 more slender and fragile than in the typical form. On the whole such specimens look so 

 different from the typical Ctenocidaris speciosa that it would seem almost incredible that 

 they could belong to the same species (compare Plate I, figs. 2, 8, 9 with Plate I, figs. 

 3-5). Nevertheless, they actually do so. Their different appearance is due to the fact that 

 they are infested with the peculiar parasitic organism Echinophyces mirabilis, which I de- 

 scribed from Rhynchocidaris triplopora, Mortensen, in my Report on the Echinoidea of 

 the German South Polar Expedition (1909, pp. 12-17, pi. xii). This parasite, the nature 

 of which is quite problematic (perhaps a Phycomycete), lives in the primary spines 

 (recognizable by some small tubes protruding through the spinules of the spine) and has 

 the extraordinary effect of causing the genital openings of the host to be reffioved from their 

 imial place in the apical system to the edge of the peristome; a new genital duct is formed, 

 leading to the pore at the peristome. It looks, indeed, as if this were a sensible action, 

 with the view of securing the transport of the eggs into the marsupium on the sunken 

 peristome where the embryos are hatched. 



In the original material of Ctenocidaris speciosa I found a couple of specimens 

 infested with this same parasite; also in these specimens the genital openings were 

 removed from the apical system, not, however, as far as the peristome, but to the middle 

 of the interambulacra. The specimens in the present collection infested with the parasite 

 give some important additional information. In this material also some of the infested 

 specimens have the genital openings in the middle of the interambulacra (Plate I, fig. 11), 

 but others have them at the peristomial edge, a little outside, or at the very edge or below 

 the edge (Plate I, figs. 10, 12). And I find that the specime?is with the genital openings at 

 the peristome are females, while those with the openings at the ambitus are males. (One of 

 the original specimens with the openings at the ambitus was also found to be a male; 

 op. cit., p. II.) 



Apart from this removal of the genital openings from the apical system the parasite 

 has no castrating effect. The infested specimens are breeding fully, and there is not the 

 slightest indication that the embryos are abnormal, though it may well be that the 

 embryos are liable to be infested with the parasite through infection from the mother. 

 I have found both eggs and nearly fully formed young ones in one and the same 

 marsupium, not, however, in many different stages of development but so that the 

 embryos were clearly of two sets, the eggs being thus evidently shed at different inter- 

 vals, a limited number (scarcely ever more than ca. 10) at a time. 



As none of the infested specimens exceed ca. 30 mm. h.d.^ (whereas the normal 

 specimens reach a size of at least ca. 50 mm. h.d.), it seems beyond doubt that the 

 parasite interferes with the growth and dwarfs the specimens. At the same time they 

 attain sexual ripeness at a much smaller size than the normal specimens. While in the 

 latter the genital openings do not begin to appear until at a size of ca. 25 mm. h.d. 

 (they are just about to appear in the specimen shown in Plate I, fig. 6), infested speci- 



^ Horizontal diameter. 



