202 ZOOLOGY. 
It consists of a narrow cesophagus (Fig. 146, w), more or 
less pentagonal near the mouth, dilating into the stomach ; 
and of a terminal intestine. The long stomach passes from 
left to right around the interior of the body, then turns up 
toward the aboral end, and curves back in the opposite 
course, again passing around the body from right to left, 
forming two series of loops partly enclosing the ovaries ;_it 
is held in place by a broad, thin membrane or ‘‘ mesentery.”’ 
The reproductive and other organs are much as described 
in the star-fish, there being five ovaries or spermaries, the 
sexes being distinct. The nervous ring around the mouth 
sends off five nerves along the ambulacra, which are accom- 
panied by a water-vascular canal sending branches to the 
tentacles, and a pseudo-hemal canal, there being an oral and 
aboral (anal) hemal ring (their presence.is denied by Hoff- 
mann), as well as an oral water-vascular ring, with five Polian 
vesicles (present only in the true Echini and Clypeastroids), 
a stone-canal and a fusiform tube or ‘‘ heart ’’* next to it, 
’ while the alimentary canal is accompanied by two hemal 
vessels, one on the ‘‘ dorsal’? and the other on the free or 
ventral side, communicating with a lacunar network in its 
walls. 
In Echinus it is difficult to perceive any bilateral sym- 
metry, the parts radiating, as in the star-fish, from the cen- 
tre; but in the Spatangus and allied forms it is easy to di- 
vide the animal into a right and left side, and the body is 
more or less elongated, as in Powrtalesia (Fig. 150), the mouth 
being situated at one end and the anus at the other. 
The mode of development of the common sea-urchin 
(Fig. 141) has been discovered by Mr. A. Agassiz. The earli- 
est stages are much as described in the star-fish. The form 
of the pluteus larva is quite remarkable, there being eight 
very long slender arms supported by slender calcareous rods 
projecting from the body, and, during the movements of 
the animal, opening and shutting like the rods of an um- 
brella. The body is provided with a sinuous row of vibra- 
* Tt should be observed that the latest and best observers are at vari: 
ance regarding the structure and function of the so-called Echinoderm 
heart.”’ 
