COMPSOMETRA SERRATA. 29 



W. B. Carpenter, 18 in describing the development of the pinnules in his 

 memoir on the development of Antedon rosaceus (p. 735), is inclined to think 

 that it takes place "rather after the manner of the joints of the stem and 

 dorsal cirri than after that of the segments of the rays and arms; that is, to 

 commence with a complete ring which extends itself longitudinally into a 

 hollow cylinder rather than a cribriform plate which wraps itself (so to 

 speak) around the extension of the sarcodic axis." This is not the case in 

 Compsometra; both arm-joints and pinnule-joints begin as a simple trans- 

 versally lying rod, from which extensions grow out to each side. These 

 extensions very soon unite and thus a cribriform plate is formed, in which 

 the original rod can be distinguished for some time, although it is not nearly 

 so conspicuous as the middle plate of the stalk-joints. It is true that the 

 joints of the young pinnule may show this original structure more distinctly 

 than the arm-joints, but this is evidently due to the fact that the rolling-up 

 of the point of the arm makes it difficult to see the structure of the young 

 arm-joints, while the young pinnule is generally straight, showing the struc- 

 ture of the growing joints distinctly. This would appear to be the usual 

 type of development of the arm and pinnule joints; at least I have found it 

 also in Isometra vivipara and essentially so in Antedon bifida. 



The first cirri are distinctly radial in position (plate xiu, figure 5); and 

 this is in accordance with what is the case in Hathrometra sarsii, as described 

 and figured by M. Sars (p. 53, tab. v, figure II); 19 also, in Tropiometra the 

 first cirri appear to be radial in position. As pointed out by M. Sars, this is 

 a remarkable difference from Antedon rosaceus (bifida), in which the first cirri 

 are stated by W. B. Carpenter to be interradial in position, which is also 

 confirmed by M. Sars (loc. cit.}. I must, however, maintain that in Antedon 

 bifida also the first cirri are really radial in position, as is also distinctly seen 

 in one of Carpenter's own figures (plate XLII, figure 3). Here four of the 

 first cirri (those with the terminal joint fully developed) are very distinctly 

 radial, while the fifth is slightly out of position, which may perhaps be due 

 to a slight error in the drawing. Upon the whole it is not quite easy to 

 determine the exact position of the first cirri; but when observed in a stage 

 while still growing (as in plate xm, figure 5) their exact position can easily 

 be ascertained. 



In the younger of the two specimens (figure 4) the stalk consists of 29 

 joints; in the larger (figure 5) of 33 joints. As the cirri are developing on the 

 upper joint of the younger specimen, the number of its stalk-joints would not 

 have been augmented ; there is thus some variation in the definite number of 

 the stalk-joints of this Pentacrinoid. Some of the upper joints, evidently 

 varying in number (3 to 5), are wider than the rest of the stalk-joints and 



18 W. B. Carpenter, Researches on the structure, physiology and development of Antedon (Comatula 

 Lamarck) rosaceus, part i, Philos. Transact., 1856. 



19 M. Sars. Mfimoires pour servir a la connaissance des Crinoi'des vivants, 1868. 



