THE ECHINODERMS. ATTACHMENT. DEVELOPMENT. 425 



both the cephalic appendages and adhesive disc taking part in the process. (Fig. 

 292, F.) In the more primitive echinoderms, such as the crinoids (Antedon), the 

 metamorphosis is even more illuminating. The larva attaches itself wholly by 

 means of the cephahc disc, as the adhesive appendages appear to be absent. Its 

 first position is with the neural, or oral surface down, as in the cypress stage of the 

 cirriped. (Figs. 274, A and 295, D.) The disc then elongates, forming a slender 

 cephalic stalk, or peduncle, and the larva turns a somersault, bringing its neural 

 side uppermost. Meantime the vestibule, or peribranchial chamber, which at 

 first is small and temporarily closed, enlarges, then ruptures, and the five appen- 

 dages project from the cup-like head, in typical cirriped fashion. (Fig. 295, G.) 

 The cirriped stage (pentacrinoid) is, however, a transitory one, and the young 

 Antedon becomes a free swimming feather-star, by the breaking down of the 

 stalk and the elongation of the appendages. 



There are many other representatives of the more modern echinoderms in 

 which the fixed stage is temporary (asteroidea) or appears to be omitted altogether 

 (ectinoidea and holothurioidea) , and the young echinoderm, after its metamor- 

 phosis, again acquires a limited power of locomotion. But in the most primitive 

 echinoderms, such as the stalked crinoids, blastoids, and cystoids, a permanent 

 attachment by an elongated cephalic stalk, in typical cirriped fashion, was the 

 almost invariable rule, and no doubt represented the primitive condition for the 

 whole class. When an echinoderm does become free, it acquires only a very 

 limited power of locomotion and of coordinated movement. Its characteristic 

 lack of efficiency in this respect is due, not so much to its simple or primitive struc- 

 ture, as to the fact that its freedom was gained at a late period in the phylogeny of 

 a very ancient group, where sessile inaction was the prevailing condition. 



Early Embryonic Development. — Let us now return to the early embryonic 

 stages to trace the beginning of the metameres, coelomic cavities, and thoracic 

 appendages. These structures, when definitely formed, have the same or a very 

 similar structure, location, and mutual relation that they have in the arthropods; 

 but they make their appearance in a somewhat different manner, and at relatively 

 different times, owing largely to the absence of yolk, which has here, as elsewhere, 

 an important influence over the method and relative rate of development. 



The Telocosle — After cleavage, which resembles that of the cirripeds, a 

 blastula is formed; from its walls many mesenchyme cells arise, comparable with 

 those which in so many arthropods wander into the yolk from the blastoderm. 

 They are usually more numerous at the point where later the so-called "gastrula" 

 infolding takes place. But this infolding is formed at the caudal end, and extends 

 forward, like the products of all teloblastic growth; and it finally closes in the anal 

 region, not the oral. (Fig. 293.) We have already seen (p. 219), that the real 

 gastrula is always developed at the head end; that the resulting endoderm grdws 

 from its point of origin backward, never forward; that the true gastrula opening 

 always persists, if at all, as the oral opening; it is always associated with the sto- 



