262 PHYL UM E CIIINODERMA. 



ciliated gastrula. In Holothuroids, Echinoids, Asteroids, 

 and Ophiuroids, it becomes quaintly modified by the 

 outgrowth of external processes, and the formation of 

 special ciliated bands. These are at first simply pre-oral 

 and pre-anal rings, but they become drawn out along 

 variously disposed and shaped processes. The larva of 

 Crinoids (of Antedori] is not so divergent. In all cases 

 the bilateral symmetry is preserved. 



The larva does not grow directly into the adult. On the 

 contrary, the adult arises, for the most part, from new 

 growth within the larva on one side. The arms or pro- 

 cesses peculiar to the larva are absorbed or in part thrown 

 off. Only in a few forms which have brood-chambers or 

 are viviparous is the development direct, and without free- 

 swimming larvae. 



The celebrated comparative anatomist and physiologist, Johannes 

 M tiller, was the first to show that the various types of Echinoderm 

 larva; might be derived from one fundamental form. 



"This fundamental type is an elongated, oval, or pear-shaped larva, 

 which is somewhat flattened on its ventral side. It has arisen from a 

 gastrula whose blastopore has become the anus, while the archenteron 

 is bent towards the ventral surface, where it communicates by the larval 

 mouth with the exterior. Besides these two apertures, the larva has a 

 third, namely, the dorsal pore of the water vascular system. The cilia, 

 with which the larva was at first uniformly covered, partly disappear, 

 and persist only in restricted regions or ciliated bands" (Korschelt and 

 Heider). 



Crinoids. The simplest Echinoderm larva is that of Antedon, a 

 somewhat modified oval, with five transverse rings of cilia (the most 

 anterior is less distinct), and a posterior terminal tuft. Eventually the 

 posterior end is elongated to form, in the pentacrinoid stage, an attach- 

 ing stalk, which is afterwards absorbed. As all the extinct Crinoids 

 are permanently stalked, there is here an instance of Recapitulation. 



Holothnroids. The larva of Holothuroids (an Aiiricularid] is much 

 quainter. Its diffuse cilia are succeeded by a wavy longitudinal band, 

 which in the pupa stage breaks into transverse rings, usually five in 

 number. The pre-oral region becomes large. 



Asteroids. Nearest the Anricnlaria is the larva of star-fishes, which 

 has the same enlarged pre-oral region. There are /wo ciliated bands, 

 of which the ad-oral is smaller, the ad-anal much larger. They are 

 extended peripherally by the development of soft bilateral arms, and 

 such a larva is known as a Bipinnaria. But another larval form in 

 Asteroids is the Brachiolaria stage, in which three warty arms are 

 formed at the anterior dorsal end, independently of the ciliated bands. 



Ophiuroids and Echinoids. In the Plutcns lame (Fig. 1 14) char- 

 acteristic of these classes the pre-oral region remains small, while the 



