Chap. II. AMBULACRA! TENTACLES. 35 



to watch. I must add, that my own observations concerning the development of 

 Echinoids and of Ophiurans have led me to an entirely different opinion from the 

 one they have expressed ; see my remarks on the embryology of Echinoderms, in 

 the Memoirs of the American Academy for 1864. 



Closing of the adinal and abadinal Areas. — Although the young Starfish has now 

 resorbed all the appendages of the. Brachiolaria (PI VI. Figs. 1, 2, 3, 4, 6, 7), it is 

 very different from the adult; the rays do not yet make a complete circuit, nor are 

 they similar to each other; the pentagon of tentacles is still open, and the first step, 

 preceding any other great change, is the closing of the actinal and abactinal areas, 

 by which the two regions are brought into their proper relations. While the arms 

 of the larva are shrinking away, the tentacular and abactinal pentagons are drawn 

 closer together by the contraction of the water-tube. The extremities of the two 

 open pentagons approach each other simultaneously by the flattening, in opposite 

 directions, of the two pentagonal spirals, until the surfaces are brought into parallel 

 planes, and the space, still separating the two ends of the pentagon (PL VI. Fig. 4) 

 gradually diminishes, when they finally join ; the Starfish is then in its normal 

 condition, and the circuit is completed, though the embryo is by no means sym- 

 metrical. 



Development of the ambulacral Tentacles of the Starfish. — While the closing of the 

 spiral goes on, the pentagon of the tentacular side is undergoing great changes. 

 We will follow these until the tentacles have acquired their normal shape, and then 

 return to the changes of the abactinal surface. The points of the inner folds of 

 the tentacular pentagon, as seen in PL V. Figs. 11, 12, 1 1 1, become rounded, 

 forming a rosette, dividing each loop into five lobes. The terminal lobe in its 

 turn goes through the same process ; two smaller lobes are developed on each side 

 of it (PL VI. Figs. 3, 5), thus dividing the original simple loop into seven lobes, a 

 terminal one (f), and three pairs (t 1 1) arranged symmetrically on the sides. The 

 first-formed lobes retain their greater size until the tentacles are well developed, 

 which at first is always in proportion to their proximity to the base of the loop. 

 The odd lobe, from which the last pair of tentacles was formed, does not participate 

 in the rapid growth of the others, and is soon outstripped by all the lobes formed 

 along the side of the original loop (PL VI. Figs. 3, 5). The point at which addi- 

 tional tentacles are formed is plainly seen in this early stage of growth ; a pair is 

 always added at the outer extremity of the arm, immediately at the base and on the 

 side of the odd tentacle (the eye-bearing tentacle), which remains at the termination 

 of the ray during the whole life of the Starfish. It is quite the reverse with the 

 additional spines of the abactinal surface of the disk; they are always formed upon 

 the disk, and are pushed out upon the arms by younger spines growing up nearer 

 the centre of the disk. This will be plainly seen when describing more advanced 



