No. 376.] REVIEWS OF RECENT LITERATURE. 275 
Without considering in detail the development of the larva, we will 
discuss certain features of the process which are of such especial 
interest as to merit the attention of the general reader. 
The ciliated gastrula which swims freely in the body cavity of the 
mother presently becomes transformed into an oval embryo without 
ciliated bands; a mouth at this stage is found upon the ventral 
surface of the body, but there is no anus; the blastopore has closed. 
A pair of ccelomic pouches are present, as well as a hydrocoel, 
which has upon its anterior face five large interradial outgrowths 
(the canals of the primary tentacles) and five smaller radial out- 
growths, each of the latter being situated at the right of one of the 
former. An adradial water canal opens upon the surface of the 
body by a dorsal pore. 
Thereupon follows a fentactula stage in which the hydrocoel, its 
extremities having united near the mid-ventral line, forms a ring; a 
Polian vesicle is present in the left-dorsal interradius ; the central 
nervous system has been established, the nerve ring having been 
derived from a thickening of the ectoderm which surrounds the 
mouth, the radial nerves, as well as nerves to the tentacles, having 
arisen as outgrowths from the circular nerve band; five pairs of 
otocysts lie external to the radial nerves at the point where they 
bend to run backward ; the mouth, situated in the center of a circle 
of tentacles, opens int6 an cesophagus lined with endoderm; the 
stomach is large; the intestine makes a single loop; an anus is now 
present at the posterior extremity of the body; a mesentery, formed 
by the fusion of the right and left coelomic pouches, attaches the 
alimentary tube throughout its whole extent to the body wall. 
The pentactula then becomes transformed directly into the larva 
with ten tentacles which, when it has attained a length of about 5 
mm., is set free from the parent. In some cases, however, young 
individuals 15 mm. or 20 mm. in length have been observed still 
within the body cavity of the mother. Birth takes place by the 
rupture of the body wall near the anus, or, more frequently, by a 
perforation through the wall of the rectum, in which case the young 
finally escape through the anal opening. 
The most striking feature of the abbreviated course of development 
of S. vivipara is the complete lack of radial water canals of the 
wall, even in the embryo. It seems to be definitely determined ' 
that in S. digitata these canals are formed in the early stages of 
1 R. Semon, Die ee der Synapta digitata und die Stammesgeschichte 
der Echinodermen. Jena. ., Bd. xxii, pp. 175-309, Taf. 6-12. 1888. 
