DEVELOPMENT OF ECHINODERMS. 249 



3. The differenliation of the typical laixial fonns. 



The celebrated comparative anatomist and physiologist, Johannes 

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

 larvre 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.) 



Criiwids. — 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. 



Hohthuroids. The larva of Holothuroids {an Aitricularia) is much 

 quainter. Its diffuse cilia are succeeded by a wa\-y longitudinal band, 



a 2 ^--v^^- - 4 



3 

 Fig. 81. — Forms of Echinoderm Larva. (After MtJLLER.) 



jn. , Mouth ; a. , anus. The dark lines indicate the ciliated bands. 



1. Supposed primitive type from which the various forms may be 

 derived. 



2. Auricularia of Holothurian. 



3. Bipinnaria of Asteroid. 



4. Pluteus of Ophiuroid. 



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

 number. The pre-oral region becomes large. 



Asteroids. Nearest the Aicriatlaria 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 Bracliiolaria 

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

 independently of the ciliated bands. 



Ophiuroids and Echiiwids. In the PInteiis larvre 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 



