36 EMBRYOLOGY OF THE STARFISH. Part I. 



conditions of the young Starfish. As the loops increase, they expand, lose their 

 character of simple folds, and soon become quite extensive sacs (t t t, PI. VI. 

 Fig. 8), opening into the main tube (/'), from which they were formed, until, finally, 

 they attain the shape represented irpon PL VI. Fig. 9. They soon grow long 

 enough to be quite movable; they contract at the base, the walls thicken towards 

 their extremity, and they become club-shaped. .The result of this contraction is a 

 change of the tentacular cavity into a rudimentary radiating tube (I"), with the 

 tentacles attached to it; it also draws together the first pair of tentacles, which are 

 usually seen in such a way as to appear like knobs (PI. VI. Fig. 5). This basal pair 

 does not lengthen so rapidly as the second pair, which in a couple of days becomes 

 the longest (PL VI. Fig. 9). Before the base of the radiating tube (f) has contract- 

 ed, the adjacent basal tentacles of adjoining loops are placed nearer together than 

 those of the same basal pair, the basal tentacles thus forming five pairs of tentacles 

 (PL VI. Fig. 8, t t), separated by the radiating tube (t"). 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 (PL 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 (f) 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 tentacles have 

 reached the state of PL VI. Fig. 9, they deyelope 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 (PL VI. Fig. 12), the clubs at 

 the extremities are supported upon comparatively narrow bases. This club-shaped 

 termination 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 (PL VI. Fig. 5), these tentacles are used by the 

 embryo in adhering to the surfaces 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. 



1 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 (PL VIII. Fig. 5). The open- 



