SITUS INVERSUS IN ECHINOIDS 137 



tion of pedicellariae on the opposite side. It seems to me 

 that they are not inhibitory to each other on the same side, 

 because they can co-exist side by side. The fact, however, 

 that in most of the double-hydrocoele larvae the pedicellariae 

 are not formed may simply be due to lack of sufficient material, 

 or that the echinus-rudiment, being more vigorous in develop- 

 ment than the pedicellariae, wins the competition. M a c B r i d e 

 assumes that ' the influences emanating from a hydrocoele 

 not only tend to inhibit the formation of pedicellariae on the 

 same side but to determine their formation on the opposite 

 side of the larva ' (15, p. 343), and that the hydrocoele can 

 act as such even in its early stage. Thus, the fact that an 

 echinus-rudiment and a pedicellaria or two can co-exist on 

 the same side is explained by him in the following manner : 

 ' If we assume that in these larvae the growth of both hydro- 

 coeles has been arrested at an early stage, but after the stage 

 at which the stimulus to form pedicellariae on the opposite 

 side had already gone forth from them, and that then, after 

 the formation of these organs on both sides had been deter- 

 mined, further nourishment became available and the left 

 hydrocoele developed further, the structure of such larvae 

 can be explained' (pp. 343-4). Kunnstrom's case that 

 some starved larvae, which had no hydrocoele, developed a pair 

 of pedicellariae (25, pp. 269-70, Text-figs. 33-5) is now very 

 difficult to understand. It is doubtful whether the hydrocoele 

 was really absent in those larvae. 



(i) Spine. — The larva of Echinus miliaris produces, 

 when fairly grown, a rudiment of a spine at the hind end 

 a little towards the right from the median line. This gives 

 rise, as do some others which develop later, to a square-ended 

 spine on the future abactinal side. This rudiment is found 

 situated a little on the left side in reversed larvae (Text-fig. 1, 

 spj), and in most of the double-hydrocoele larvae, in an almost 

 median position. Such a different position of this spine is 

 undoubtedly correlated with the different behaviour of the 

 echinus-rudiment. Characteristic are the spines which develop 

 in the larvae devoid of hydrocoele (Case 27). As already 



