44 EMBRYOLOGY OF THE STAEFISH. 



(PI. VI. Fi(j. 8, i t), separated by the radiating tube (/"). In proportion 

 as the tentacles elongate, the separation between them and the radiating 

 tube is more distinct, and very soon the tentacles appear like club-shaped 

 branches projecting from it (PI. VI. Fig. 9) ; the first pair of tentacles 

 are somewhat shorter and stouter than the second, which is the longest, 

 while the three terminal tentacles have nearly the same size, the odd 

 tentacle {t') not showing as yet the slightest tendency to become club- 

 shaped, though developed so much earlier than the larger basal pairs at 

 its base. 



Formation of the Sucker of the Tentacles of the Starfish. — When the ten- 

 tacles have reached the state of PL VI. Fig. 9 they develop rapidly ; the 

 walls at the extremity of each tentacle thicken so much, that the cavity 

 becomes a pointed tube set into a somewhat conical head, which grows 

 more club-shaped, and projects beyond the walls of the tentacles as they 

 increase in length, so that, when the basal pair of tentacles equals again 

 in length the second pair (PI. VI. Fig. 12), the clubs at the extremities 

 are supported upon comparatively narrow bases. This club-shaped termi- 

 nation is the future disk of the tentacle, the sucker, by means of which 

 the Starfish adheres so firmly to rocks. From an early period, even 

 when there is only one large pair of tentacles at the base of the ray, 

 and when the others exist only in the most rudimentary condition (PI. VI. 

 Fig. 5), these tentacles are used by the embryo in adhering to the sur- 

 faces upon which it is placed ; and, though they are not provided with a 

 regular sucking disk, they fasten themselves so firmly, by means of these 

 loops, that it requires considerable force to make them loose their hold. 



Formation of the Eye. — We have seen that, unlike the others, the odd 

 terminal tentacle does not become club-shaped, but increases slowly in 

 length alone, the walls retaining a uniform thickness. It is not till all 

 the pairs of tentacles are well developed that we begin to perceive slight 

 changes (PI, VIII. Fig. 5). The opening leading into the radiating canal 

 contracts, the basal portion of the tentacle swells, and it assumes a some- 

 what pear-shaped form, the swelling at the base increases, principally on 

 the oral side, and we soon trace in it an accumulation of pigment cells 

 (PI. VII. Fig. 6, e), which, by the time the other tentacles have developed 

 knobs, and equal in length the diameter of the arms, has become a brill- 

 iant carmine spot (PI. VI. Fig. 12, e; PI. VII. Fig. 6, e, and PI. VIII. 

 Fig. 5, e). This odd tentacle, placed at the extremity of the radiating 



