DEVELOPMENT OF ECHINODERMA TA. 221 



rounding space "between endoderm and ectoderm. As they have a 

 common origin, the outgrowth or outgrowths which give rise to enterocoel 

 and hydrocoel may be, and often are, termed vaso-peritoneal. 



There is not perfect agreement as to this united origin, but the 

 following facts are generally recognised. 



In Holothuroids there is a single outgrowth which gives rise to 



both body-cavity and water-vascular system. 

 In Echinoids, Asteroids, and Ophiuroids, there are two out- 

 growths, from the left of which the water-vascular system 

 arises. 

 In Crinoids [Atiiedon), there are three outgrowths, that which 

 gives rise to the water-vascular system being independent of 

 the pair which form the body-cavity. 

 In most cases a dorsal pore bringing the hydroccel into com- 

 munication with the exterior has been detected. 



3. The differentiation of the typical larval forms. 



The celebrated comparative anatomist and physiologist, Johannes 

 Miiller, was the first to show that the various types of Echinoderm 

 larvae 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 an anterior terminal tuft. 



Holothuroids. The larva of Holothuroids (an Auricularia) 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 Auricularia is the larva of starfishes, which 

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

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

 extended peripherally by the development of soft arms, and such a larva 

 is known as a Bipinnaria. But this may be succeeded by a Brachiolaria 

 stage, in which three warty arms are formed at the anterior dorsal end, 

 independently of the ciliated bands. 



Ophiuroids and Echinoids. In the Pluteus larvse characteristic of 

 these classes, the pre-oral region remains small, while the post-anal 

 region becomes large. There is one undulating ciliated band, the 

 course of which is much modified by the growth of six long arms, with 

 temporary calcareous supports. This quaint form is often compared to 

 a six-legged painter's easel. 



