DEVELOPIMEXT OF ANTEDON (COMATIJLA, LAMK.) EOSACEUS. 731 



descend the stem, a gradual passage from such as consist of little else than the primor- 

 dial ring, to those which have attained their complete cylindrical form and their full 

 dimensions. Moreover it is obviously there that the nutritive activity will be the 

 greatest ; the sarcode-substance intervening between the base of the calyx and the sum- 

 mit of the stem being in most direct relation with that of the interior of the calyx 

 (through the imperfect closure of its basal plates, § Gl), and consequently with the 

 visceral apparatus. And there is no other part of the stem in which there is the least 

 appearance of any multiplication of segments, either by the subdivision of those akeady 

 formed, or by the interpolation of new ones ; — the suggestion of Professor Allmax, that 

 the transverse ridges running round the centres of the segments may be the indication 

 of such a division, being negatived by the facts already stated as to its real character. 

 That this ridge, which is prominent in the segments of the upper and middle part of the 

 stem, so far disappears in those of the lower as to be represented only by a single line, 

 is due to the circumstance that the lower segments are those of which the skeleton is 

 first completed. — By the time that the opening-out of the Calyx commences in the man- 

 ner to be presently described, the number of segments in the stem has usually risen to 

 15 or 16 ; those of the inferior third of the stem are pretty nearly solidified throughout, 

 only a small passage still existing through their interior ; but those of the middle and 

 upper thirds of the stem are still so far from having attained their completion, that 

 their calcareous cylinders when broken across are found to be mere shells. The highest 

 plate, on which the base of the calyx rests, is now distinguished from those below it by 

 its somewhat larger diameter ; but it does not as yet present any approach to the pecu- 

 liar shape which it afterwards comes to possess. — The entire stem remains clothed with 

 a thin layer of sarcodic substance ; and its cavity is occupied by a cylinder of the same, 

 which forms a continuous axis throughout its entire length, and passes up at its summit 

 into the calyx. I have not been able to see any traces, at this early stage, of that fibrous 

 structure which may be distinguished about the ends of the segments at a subsequent 

 time. 



64. A very important change now takes place in the relations of the several parts of 

 the Calyx and its contents, which gives to the body of the more advanced Pentacrinoid a 

 much closer resemblance to that of the adult Anfedon. For instead of being completely 

 included within a calcareous casing, which not only supports it below, but can close over 

 it above, the visceral apparatus which occupies the cavity of the calyx is henceforth to 

 be merely supported by its skeleton ; its upper surface losing all protection except such 

 as is aff"orded by the infolding of the arms, and being extended into a disk of which the 

 mouth only occupies the centre. This change is essentially connected with the increased 

 development of the intestinal tube, which now forms a nearly complete circle around the 

 stomach, and comes to possess a second or anal orifice. When we examine the compo- 

 sition of the calyx (Plate XXXIX. fig. 1, c, and fig. 3), we find that the original basals 

 have undergone little if any increase, but that iivcjirst radials (r', r') are now much 

 larger, and spread out so as to extend the base of the cup, instead of merely forming 



