IN THE MOKPHOLOGY OP THE CYSTIBEA. 27 



The fourth opening of Aristocystis (d) is situated immediately 

 behind the elongated peristome, generally towards its left end ; 

 though sometimes it is a little nearer the centre (PL I. figs. 12, 13) . 

 The position of the third or genital opening (c), however, seems 

 to vary considerably in this type as depicted in Barrande's figures. 

 It is sometimes on the very edge of the anal opening (6), as shown 

 in fig. 12, while in other individuals it is nearly halfway towards the 

 peristome (fig. 13), as in Protocrinus oviformis. The former con- 

 dition obviously suggests that in other Cystids, in which no third 

 opening has been discovered, the genital ducts and rectum may 

 have opened together into the space beneath the valvular pyramid? 

 through which they would have had a common outlet to the 

 exterior, analogous to the " anal spiracle " of the Blastoids. 

 Yolborth*, de Yerneuilf, and EoemerJ long ago suggested 

 this as a possible explanation of the function of this structure, 

 and we now know of a precisely similar case among certain 

 Starfishes. 



In the members of the family Pterasteridae there is a sort of 

 marsupial pouch on the, dorsal surface of the disk, into which the 

 oviducts and the anus both open, and it communicates with the 

 exterior by an opening which Sladen has termed the oscular 

 orifice. In the genera Symenaster and PytJionaster this opening 

 is guarded by five fan-like valves §, each composed of a number 

 of spines united by perisome ; but Avhile in Hymenaster the mar- 

 supium or nidamental cavity covers the entire disk, owing to the 

 great development of the supradorsal membrane, it seems to be 

 reduced in Pythonaster to the small space within the oscular 

 valves. Many Cystids were probably in this condition, i. e. with 

 a valvular osculum common to the oviducts and rectum — e. g., 

 Agelacrinus, Amygdalocystis, Gomarocystis, Caryocrinus, Semi- 

 cosmites, Maloeystis ; though it is of course possible that they 

 may have had a separate genital opening which has not yet been 

 discovered. In eleven of the twenty-one genera in which the third 

 opening has been described, whether as the anus or as the genital 

 pore, it is situated behind the mouth in the same interradius as 



* Bull. Acad. Imp. Sci. St. Petersbourg, 1842, tome x. p. 295. 



t ' Geologie de la Russia d'Europe et des Montagnes de I'Oural,' par Mur- 

 chison, de Verneidl, et Keyserling : Londres et Paris, 184.5, vol. ii. p. 27. 



X ' Letliasa Geognostica,' Bd. i. p. 263. 



§ Zool. Chall. Exp., " Eeport on the Asteroidea," vol. xxx. 1889, p. 469, 

 pi. Ixxxiv. figs. 1, 3, pi. xcT. iig. 1. 



