MUSEUM OF COMPARATIVE ZOOLOGY. 125 



genital openings, no, no, one natural, the other partly cut away. The central cavity 

 above the mouth is the stomach proper, from which radiate pouches, Stf, more simple 

 than in Gorgonocephalus, and adherent above and below on ten radiating lines, as 

 may be seen under the radial shields, I, I. The genital organs were wholly unde- 

 veloped, and there were to be found only the ten radiating compartments of the body- 

 cavity, whose lining membrane, as in Gorgonocephalus, doubtless takes on the 

 reproductive function. They were continued from the genital openings, n o, under 

 and between the pouches of the stomach, and intercommunicated by a ring-tube 

 round the mouth, sections of which are seen at 5/, 5/. — r, mouth-tentacles; 

 d", teeth. 



Fig. 6. f . A vertical, radiating section through an arm, a little beside the median 

 line, and a part of the disk of Astrocnida isidis. The type is Astrophyton-like, with 

 Ophiuroid features. The stomach has pouches, St', less folded and complex than 

 those above described ; and it is quite free on the under side. "Where the stomach 

 proper folds back from and over the mouth-wall, the two adhere to form a ring-tube 

 (perihcemal canal) just outside the stomach-sphincter, clio. This and the -want of 

 attachments along the floor of the stomach are characters of Ophiurans. The ova- 

 ries, 5, consist of egg-clusters, and lie in the body-cavity, into which penetrate the 

 genital openings, as in Gorgonocephalus. — d, mouth-papillae ; /, section of mouth- 

 frame ; r, mouth-tentacles ; iif, passage for the nerve, bloodvessel, and water-tube of 

 the arm ; lo', arm-bones. 



Fig. 7. 1. Ophiocreas cedipus, male. Base of an arm tipped a little from the ob- 

 server, with outer corner of the disk, whose side is cut away, while the integument of 

 the arm is cut and folded back, showing a double-lobed spermary, 5, 5, whereof there 

 is one on either side of the upper surface of the arm. The pleated floor of the stom- 

 ach, St, St, is slit to expose the genital pouch, or bursa, below, which is an indenta- 

 tion of the disk-wall, debouching by the genital opening, n o. The spermary is en- 

 closed by a thin continuation of the lining membrane of the disk, and at its inner 

 end connects by a pore with the bursa. Under the outer end of the radial shield, I, 

 may be seen a portion of the stomach, which adheres not only to the roof of the 

 disk-wall, but also to the wall of the lower interbrachial space, so that the true body- 

 cavity is reduced to a sinus over each arm, and to the closed ring-tube (perihaemal 

 canal) about the mouth. 



Cambridge, February, 1881. 



