DEVELOPMENT OF ECH1N0DERMS. 243 



not purely a "packing tissue," but may acquire a distinctly 

 epithelial character. Many of the early mesenchyme cells 

 are calciferous, combining to form the larval skeleton. 



The larva is, first of all, a slightly modified, diffusely 

 ciliated gastrula. In Holothuroids, Echinoids, Asteroids, 

 and Ophiuroids, it becomes quaintly modified by the out- 

 growth of external processes, and the formation of special 

 ciliated bands. The larva of Crinoids (i.e. of Antedon only) 

 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. The structures peculiar to the larva are 

 absorbed or in part thrown off. Only in a very few cases is 

 the development direct. 



The details of the development are so difficult that we can here only 

 give a few notes. There is a close connection between the origin of the 

 body cavity and that of the water vascular system. Both are the results 

 of an outgrowth or of outgrowths from the gastrula cavity or archen- 

 teron, into the surrounding space between endoderm and ectoderm. As 

 they have a common origin, the outgrowth or outgrowths which give 

 rise to enteroccel and hydroccel may be termed vaso-peritoneal. 



The celebrated comparative anatomist and physiologist, Johannes 

 Muller, 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 a posterior terminal tuft. 



Holothuroids. — The larva of Holothuroids (an Auricularid) 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 



