276 PHYLUM ECHINODERMA. 
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 Anfedon) 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 
Fic. 141.—Stages in development of Echinoderms.—After Selenka. 
1. Section of blastula of Synapta digitata (Holothuroid), with a hint of 
gastrulation. 2. Section of gastrula of 7o.xopneustes brevispinosus (sea- 
urchin) ; ec., ectoderm ; ez., endoderm ; 7., segmentation cavity with 
mesenchyme cells init. 3. Section of larva of Asterina gibbosa (star- 
fish) ; BZ., blastopore ; g., archenteron; v.Z., vaso-peritoneal vesicle ; 
yr. and Z., right and left sides. 
are viviparous is the development direct, and without free- 
swimming larvee. 
The celebrated comparative anatomist and physiologist, Johannes 
Miiller, was the first to show that the various types of Echinoderm 
larvee 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, 
a persist only in restricted regions or ciliated bands” (Korschelt and 
eider). 
Crinoids.—The simplest Echinoderm larva is that of Azedon, a 
somewhat modified oval, with five transverse rings of cilia (the most 
