ot 
ECHINODERMS. 137 
and the creature having lost its apparatus for motion sinks to the 
bottom: tentacles in the meanwhile, five on each side of the mid- 
line, have been developed, and hooks are seen at the two extremities 
of the body which shew by their peculiar form that those extremities 
correspond to the ends of the arms: the embryo now cup-shaped 
from the increased convexity of its dorsal surface attaches itself to 
the bottom by this surface from which it secretes its pedicle. The 
absence of symmetry in the relation of the Echinoderm to its larva 
is indicated by the stem of the Echinoderm being placed at right 
angles to the axis of the larva, and the tentacles and mouth on the 
opposite surface!. 
In by far the greater number of Echinoderms the embryos pass 
gradually into forms which, however remarkably they may differ, 
are all laterally symmetrical. The axis becomes bent and on the 
ventral surface (that where the mouth opens) is a depression bounded 
above and below by transverse bands of cilia which are continuations 
of the lateral bands which bound the dorsal surface. They all have 
a complete digestive tube consisting of moufh, cesophagus, stomach, 
intestine and anus. This tube is placed in the median plane, the 
mouth in the ventral depression described above, and the tube 
curves from it to terminate beyond the transverse band of cilia 
above the mouth on the same ventral surface. Also they have all 
an aquiferous system, a tube terminating externally in a dorsal pore 
and internally in a sac. When Muetter observed the singular 
forms of the larvae of Ophiure and Echini with their long processes 
supported by slender rods of carbonate of lime he named them 
Pluteus from their general resemblance to a painter’s easel with his 
work upon it. In Asteriw and Holothurie the larve have a more 
flattened form, like a coat of arms with its surrounding ornaments. 
The process of development in these different larval forms is two- 
fold. In the first case the body of the Echinoderm is formed by 
gemmation round the stomach of the larva, which continues to be 
ats stomach, and when it is formed, all that remains of the larva, 
with the exception of certain structures in connexion with the aqui- 
ferous system is gradually (Ophiura and Echinus) or simultaneously 
(Bipinnaria asterigera) vejected. In the second case the symmetrical 
1 (Comp. Beobach. uber Anatomie u. Entwickelung einiger wirbdlosen Scethiere von 
Dr. W. Buscu. fol. Berlin, 1851, s. 82—88.] 
