70 THE VOYAGE OF H.M.S CHALLENGEE. 



One of these supplies most of the arms in the anterior division of the right ray, and tlie 

 other most of those in its posterior division. But some of the arms in both divisions 

 receive no groove at all ; and there is only a faint line on the arm-bases indicating the 

 course of the water-vessel which proceeds to the hindermost arms of this ray, while 

 there is a similar line on the eastern side, indicating the course of the water-vessel 

 suppljdng the hinder arms of the left ray. [Compare fig. 3 on p. 92.] 



The posterior ray, however, has no separate ambulacrum of its own upon the disk ; 

 and the right limb of the usual horse-shoe shaped curve is therefore incomplete. At the 

 point where the large and widely open primary groove-trunk forks into the two divisions 

 of the right lateral ambulacrum, a faint line may he seen starting from its inner side. 

 This is all that there is to represent the posterior ambulacrum. It curves backwards 

 round the margin of the disk to its hinder edge, gradually becoming less and less distinct 

 the farther it goes, until its ultimate branches to the arm-bases are only traceable with 

 the greatest difiiculty. All the arms of this ray are therefore unprovided with ambulacra, 

 and with most of the structures which are correlated with their presence (PL LXI. fig. 3). 

 They have neither blood-vessel nor ventral nerve, and their water-vessels are simj)le 

 tubes like the integumentary water- vessels of the Molpadidse. 



These variations in the development of the ambulacra on the disk and arms of 

 Actinometra have somewhat important morphological bearings, as will be explained in a 

 later chapter. Curious malformations of the disk are occasionally to be met with among 

 the Crinoids. Thus in the disk of Metacrinus angulatus, which is represented on 

 PI. XXXIX., there are two anal tubes of unequal size, between which I found a Myzoatoma 

 reposing. Three monstrosities of Comatula disks have also come under my notice. In 

 one case, an Antedon, there are two anal tubes as in Metacrinus angulatus, but of more 

 equal size ; while in another, Actinometra stelligera, there are not only two anal tubes, 

 but also two mouths (PI. LVI. fig. 8). The true mouth is interradial, and there is a 

 large sub-central anal tube immediately behind it, as in all species of this genus. But 

 there is also a second anal tube occupying the interval between the posterior ambulacrum 

 which curves round the western portion of the disk, and the branches of the lateral 

 ambulacrum on its eastern side. This is very irregular in its character, and expands at 

 one portion of its course into a second peristome in which is a small mouth. Lastly, in 

 a specimen of Antedon rosacea from Milford, only four ambulacra reach the central mouth, 

 while the fifth (tlie left antero-lateral) has a second mouth all to itself (PI. LVI. fig. 6). 



A. The Okal Plates. 



The well-known oral plates which are so characteristic a feature in the Pentacrinoid 

 larva of Antedon and Actinometra are resorbed before maturity is reached. But they 

 persist through life in Holopus, Hyocrinus, Rhizocrinus, and Thaumatocrinus, imme- 



