DEVEL0P5IENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 733 



good deal of variation. I have counted as many as 24, and as few as 16 ; the average 

 may be considered about 20. 



60. Soon after the highest segment of the Stem begins to enlarge, we notice on that 

 portion of its under surface which extends itself beyond tlie segment whereon it rests, 

 one or more minute tubercles, which are the origins of the dorsal cirrhi. Each of these 

 tubercles is formed by a projection of the sarcodic substance of the perisomc, within 

 which are observable one or more minute annular disks of calcareous reticulation. The 

 projection of the tubercle gradually increases, and the number of disks (which are the 

 rudimental segments of the dorsal cirrlii) is multiplied ; so that each incipient cirrhus 

 presents the form of a short cylinder marked by transverse annulatious (Plate XL. figs. 

 1, 2, cir). The length of this cylinder is progressively augmented by the formation of new 

 disks, and by an increase in the thickness of the earlier ones ; and the terminal segment 

 soon presents an mdication of the peculiar character it is ultimately to assume. Hence 

 it is obvious that the new segments cannot be added at the extremity of the tentacle ; 

 and since, during the whole progress of its growth, we always find that the basal segment 

 is tlie shortest, it seems most probable that the increase in the number of segments is 

 effected by the interpolation of new segments at the point at which the cirrhus springs, 

 from the plate which bears it, — a conclusion which is conformable to what has been 

 already stated (^ 29) of tlie relation of the cin'hi to the peculiar modification of the 

 central axis contained within the centro-dorsal basin. — As each cirrhus elongates itself, 

 its extremity, which was at first bluntly rounded, becomes pointed, the terminal segment 

 developing itself into a conical form, though still covered with the same thick invest- 

 ment of condensed sarcode as extends over the entire length of the rudimentary cirrhus'.r 

 — The exactness with which radial spnmetiy is maintained throughout the formation of 

 the skeleton of this organism (save in the case of the Anal jilate, which has special refer- 

 ence to the -sdsceral mass), would lead us to anticipate that the first five cirrhi would be 

 put forth together, radiating from the central tubercle, and developing themselves at 

 correspondmg rates. This, however, is not the case, for they are developed succession- 

 ally, the first of them usually exhibiting numerous segments with a conical termination 

 by the time that the fifth makes its appearance (Plate XXXIX. fig. 1, d). The cirrhi of 

 the first whorl alternate in position with the rays, so that one of them is opposite to the 

 anal plate ; and this I generally (though not always) find to be the latest in its develoj)- 

 ment, the most advanced being the one which corresponds in position to the couimence- 

 ment of the intestine. 



67. The development of the Arms continues to take place on the plan already described 

 in detail by Professor Wrs'iLLE Thomson. Each of them is terminated by a " growing 

 point" of condensed sarcode, in which new segments successionally originate (Plate 

 XXXVIII. fig. 1), a cribriform calcareous film bemg first formed on the dorsal face, and 

 an ingrowth of fasciculated calcareous tissue then taking place in such a manner as in the 

 first instance to leave a deep groove on the ventral face, which is afterwards converted 

 into a canal by the closing-over of its margins ; so that the transverse section whicli at fii'st 



