432 



ZOOLOGY 



SECT. 



modified by the formation, except in the Crinoidea, of variously 

 arranged processes along the course of the peri-oral band. The 

 resulting larva, echinopczdium or dipleurula, always exhibits 

 marked bilateral symmetry. It has a pre-oral lobe on which an 

 apical plate comparable to that of the trochophore (p. 322) may 

 be developed. 



In the Asteroidea the larva is either a bipinnaria (Fig. 343, 

 4 to 6) or a brachiolaria. The former has a series of bilaterally 

 arranged processes or arms ; the latter has, in addition, three 

 processes not developed in the course of the ciliated band arid 

 used for fixation. The larva of Asterina, the development of which 



has been described and illustrated 

 on pp. 388-393, is a greatly modified 

 bipinnaria with the pre-oral lobe large 

 and eventually serving as a stalk, and 

 the pre-oral band of cilia confined to 

 the edge of the larval organ and de- 

 void of the bilateral processes of 

 the normal bipinnaria. The bipin- 

 naria is usually free-swimming, but 

 sometimes, as in the case of Asterina 

 (p. 391), creeps on the surface of a rock 

 by means of the pre-oral lobe, and 

 subsequently becomes fixed by means 

 of the latter modified to act as a 

 stalk. In at least one form the bipin- 

 naria, developed in a brood-pouch, 

 adheres to the parent by means of the 

 pre-oral lobe which takes the form of 

 a short stalk. In both the Ophiu- 

 roidea and the Echinoidea (Fig. 343, 

 7 to 9) the larva has the form 

 which is known as the pluteus. The 

 pluteus has a series of slender arms 

 directed forwards and supported by a skeleton of delicate calcareous 

 rods. The larva of the Holothuroidea, the auricularia (2 and 3), 

 has a number of short processes developed in the course of the 

 ciliated bands ; subsequently, in the pupa stage, the ciliated bands 

 become broken up into a series of ciliated hoops encircling the 

 body. Of the Crinoidea the development of Antedon alone is 

 known. Blastula and gastrula stages occur as in the Starfish, 

 but the history of the archenteron and its diverticula is widely 

 different, though the outcome is the same viz., the differentiation 

 of a primitive enteric canal, an anterior coelome, from which a 

 hydrocoele becomes separated off, and a pair of coelomic sacs. 

 The larva (Fig. 344) becomes barrel-shaped, and the pre-oral lobe, 

 which is not very conspicuous, develops an ectodermal thickening 



FIG. 344. Free-swimming larva of 

 Antedon, from the left side. 

 I-V, ciliated bands ; bai to ba^, 

 the five basals ; ori to or 5 , orals' ; 

 1, vestibule ; 2, intestinal vesicle ; 

 3, right enterocoele ; 4, calcareous 

 joints of the stalk ; 5, pedal plate. 

 (From Lang, after Seeliger.) 



