130 E. W. MACBRIDE. 



mata as an object for this study. It has both advantages and 

 disadvantages. The former are (1) that it breeds the whole 

 year round ; (2) that it retains the young in its genital bursae 

 till a very late stage of development ; and (3) that the adult is 

 of such a small size as to render it a very favorable object 

 for microscopic study. Hence it will be understood that one 

 can always obtain an unbroken series of stages ; and those 

 phases frequently occur which in other Echinoderms are most 

 difficult to obtain, viz. those immediately following on the 

 close of the larval period. On the other hand, the great dis- 

 advantages are (1) that the Ophiurids are probably the most 

 modified group of the Echinoderms; (2) that the development 

 of Amphiurasquamata, being more or less direct, is likely 

 to be somewhat altered ; (3) that the embryos are excessively 

 small; my earliest post-larval stages having a diameter of only 

 about '2 mm., whilst the unsegmented eggs of Antedon 

 rosacea have a diameter of '3 mm. 



With regard to the second disadvantage mentioned above, 

 one must remember that a larval form can be recognised 

 though much simplified in comparison with the ordinary 

 Ophiurid Pluteus ; and that the organs treated in this paper 

 are less likely to be modified in their development, since they 

 are all post-larval. The small size of the embryos forced me 

 to give up the attempt to investigate the origin of some of the 

 sinuses, such as the '' radial perihsemal " and the " perihaemal 

 ring sinus. '^ So also I can make no statements with certainty 

 as to the origin of the subneural sinus ; except this, that 

 Cuenot (6) is most certainly wrong when he asserts that it is 

 formed by an involution of the ventral ectoderm of the arm. 

 As a matter of fact, the nerve- cord has attained its deep posi- 

 tion from the time of the very first appearance of the arms, 

 and the figure which Cuenot gives has no counterpart in 

 nature. All the sinuses mentioned are at first exceedingly 

 narrow slits, considerably less in diameter than the nuclei of 

 the surrounding tissue; and as the cell plasma of Echinoderm 

 mesoderm is difficult to stain, one can realise how easy it is to 

 be mistaken about the extent and communications of such 



