398 



ZOOLOGY 



SECT. 



case of Asterina (p. 361), creeps on the surface of a rock by means 

 of the larval organ. In at least one form the Bipinnaria, devel- 

 oped in a brood-pouch, adheres to the parent by means of the 

 larval organ which takes the form of a short stalk. In both the 

 Ophiuroidea and the Echinoidea (Fig. 315, 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. A remarkable feature of the Pluteus 

 in one case at least (Ecliinocyamus pusiUus) is that the ciliated 

 bands consist of rows of flagellate collared cells, similar to those 

 of the endoderm of Sponges. 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 



FIG. 316. Development of Antedpn. A, larva with ciliated band, posterior tuft of cilia, and 

 mouth (to the right) ; B, larva with the developing plates of the peiitacrinoid stage ; C, penta- 

 crinoid stage. 



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, and has been already described (p. 377). 

 The larva (Fig. 316) is barrel-like, with four transverse bands 

 of cilia and a bunch of stronger cilia at the posterior end. The 

 posterior end of the larva becomes drawn out to form a narrow 

 process or stalk, by means of which it becomes attached to some 

 foreign object. The attached condition of Antedon, termed the 

 pentacrinoid stage, is only temporary; as the. development ap- 

 proaches completion, the stalk is absorbed, and the Feather-Star 

 becomes free. 



In the transition from the bilateral larva Pluteus, Bipinnaria, 

 Brachiolaria, or Auricularia to the radial adult there is a marked 

 metamorphosis. As the adult form becomes developed on one side 

 of the larva, the larval arms or processes become absorbed. In the 



