REPORT ON THE OPHIUROIDEA. 279 



Ophiocreas. 



Ojihiticrea*, Lym., Bull. Mus. Comp. Zool., vol. i., part 10, 18G9. 



Disk very small, slightly arched, and divided into radiating lobes by the radial 

 shields, and covered by a soft skin. Large strong teeth in a single vertical row. No 

 mouth papillae or tooth papilke. Arms simple, very long and slender, and covered by a 

 soft skin, which obscures the underlying parts ; their under side is nearly covered by the 

 side arm plates which bear two slightly rough cylindrical tentacle scales (or arm spines), and 

 are continued upward by a row of narrow plates homologous with tipper arm plates and cover- 

 ing sides and top of arm. Under arm plates small but distinct. Two small genital open- 

 ings in each interbrachial space, slanting or nearly vertical and placed at the outer corners. 



There are five pairs of side mouth shields, each with' its mouth shield, apparently 

 always madreporic, for each has a tube which I have not yet properly traced, but which, 

 doubtless, is a stone canal. These shields are an Ophiuran character shared by Astroschema, 

 which is only an Ophiocreas with a granulated skin. 



Ophiocreas brings us a long step towards the true Ophiurans. An opening, somewhat 

 inclined from the vertical, through the base of an arm and the outer corner of the disk is 

 sketched in (PL XL VI. fig. 1). The integument of the arm, cut through on the side, is 

 lifted and thrown back, whde the side of the disk is wholly cut away. Above the arm 

 bones at the base of the arm lie the double lobed spermaries (S,S), long, cylindrical, 

 smooth bodies, a little curved, and tapering at each end. On the opposite side of the arm 

 lies a corresponding pair. The genital opening (no) enters a spermatic pouch, or bursa, 

 separated from the body cavity, as in Ophiurans. An extension of the lining membrane 

 of this bursa encloses the spermatic lobes ($,<$), which discharge into it by a pore at their 

 inner end. I have already remarked 1 that the ovaries of this species lay in the same 

 position, at the base of the arm. I made, however, a mistake as to the " large eggs which 

 are about "7 mm. long." They are not eggs but clusters of eggs, each wrapped in its 

 membrane and comparable to those of Astrogomphus. The position of the genital 

 organs, though curious, is not so exceptional as might at first appear. Among true 

 Ophiurans the space between the digestive cavity and the sides and roof of the disk wall 

 is crammed with these organs when gravid. In Ophiocreas, however, not only is the 

 disk small, but its body cavity is limited to the perihsemal canal and to a sinus over each 

 arm. Everywhere else the digestive cavity adheres to the body wall ; therefore the genital 

 organs are, as it were, forced into the space between the skin of the arm and the arm bones. 



The dissection of a female Ophiocreas (an undescribed species from the " Blake " 

 dredgings) demonstrated the homology of the genital organs with those of Ophiurans. 

 There were two long lobes, or tubular membranous bags, on either side of the upper 

 surface of the arm. These were in process of discharging their eggs, which takes place 

 by the breaking up of the egg clusters and the passage of the eggs to the inner end of the 



1 Bull. Mus. Comp. Zool., vol. vi., part 2, p. 60. 



