MUSEUM OF COMPARATIVE ZOOLOGY. 179 



" Bulletins," seems to point to the same conclusion ; for the lower ring 

 of pentagonal (or rather hexagonal) plates must surely be the second 

 radials, and the triangular ones above them the axillaries. 



A fragment of a living Holopus was dredged by the " Blake " in 120 

 fathoms, off Montserrat (No. 157). It was preserved in spirit and sent 

 over to Sir Wyville Thomson, who asked me to cut some sections of the 

 arms for him. The condition of the dried individuals hitherto known 

 had led him to suspect " that the tissues are very imperfectly differenti- 

 ated, almost protoplasmic. When an arm is put into boiling water it falls 

 to pieces at once, the joints simply coming asunder, and showing no trace 

 of muscular or other organic connection except the axial cords of the 

 joints, which sometimes keep two joints hanging in connection for a little." 

 The spirit specimen, however, told a different tale altogether, and the 

 sections which I have made from it show that the soft parts of Holopus 

 differ but little from those of any ordinary Criuoid. The arm-joints are 

 articulated by means of muscles and ligaments in the usual way. The 

 two large arm-canals, the coeliac and the subtentacular, are separated by 

 a smaller genital canal containing the genital cord. This has exactly 

 the same structure as that of any common Antedon, and the ovaries 

 which it bears at intervals are much more like tho^Q o^ Antedon eschrkhtl 

 in their histological structure than are those of manv Comatidte. The 

 ambulacral groave is quite narrow in proportion to the breadth of the 

 arm, and the ovaries extend but a very little way into the pinnules. 

 The branches which leave the axial cords of the arms to supply the 

 pinnules take a somewhat singular course. For they are thrown into 

 loops in a dorsoveutral direction, which are small at first, immediately 

 beneath the arm-canals, but become much more marked at the bases of 

 the pinnules, within which the cords still retain an undulating course. 



Above the water-vessel is the usual darkly colored ambulacral epithe- 

 lium, which is doubtless separated from the water-vessel by the ambu- 

 lacral nerve and bloodvessel, though I have not been able to see them 

 clearly. There are no large and imbricated reniform plates at the sides 

 of the ambulacra, such as occur in Iikizocrmus, Bathijcrinus, and Hi/o- 

 crinus. But the tentacles are unusually large, and taper rapidly from a 

 broad base ; while the lower thick part of the shaft of each tentacle 

 is protected by a well-developed calcareous reticulation, above which 

 are groups of more or less closely united spicules. 



Eton College, September, 1882. 



