DEVELOPMENT OF CUCUMARIA ECHINATA 219 
internal (axial) side between the hydrocoele and peritoneal 
epithelia. 
In C. planci Ludwig (22) observed the first appearance 
of muscle fibres in tentacles on the seventh day (p. 612), in 
pedicels on the tenth day (p. 185), and along the mid-ventral 
radial canal on the thirteenth day (p. 182). According to him, 
all these are derived from the hydrocoele epithelium. 
Nervous System.—The nervous tissue is well marked 
in this stage. Immediately below the atrial cavity the ring 
nerve 18 formed, encircling the still closed anterior end of the 
gut. Anteriorly a branch, the tentacular nerve, is put forth in 
each interradius to run along the oral side of the tentacle. 
Posteriorly the five radial nerves appear, of which the mid ventral 
is the strongest. The latter gives out a pair of branches to 
the primary pedicels. 
Along the oral side of each tentacle, a part of the atrial cavity 
extends backwards as a thin flat canal. On reaching the ring 
nerve, these canals unite with one another to form a circular 
canal above the former. This is the epineural ring. From 
this the epineural canal is sent out along each radial nerve 
(Text-fig. 4,8, c,ep; Pl. 9, figs. 24, an, enc). 
I can give no further account, and will refer to Lud wig’s 
detailed descriptions on the origin, differentiation, and develop- 
ment of the nervous system given for C. planci (22). Accord- 
ing to him, the rudiments of the nervous system first 
appear on the fourth day (p. 608), the epineural ring and 
canals are formed on the fifth day (p. 609), differentiation of 
fibrous structure takes place on the sixth day (p. 611), the 
tentacular nerves are formed on the ninth day, and the pedal 
nerves are given off on the seventeenth day (p. 188), In C. 
echinata I could make out all these features even in the 
fifth-day larva. 
Calcareous Deposits.—In my culture calcareous 
deposits made their first appearance on the sixth day. They 
occur at three places, i.e. in the wall of the axial sinus (Text- 
fig. 5, mp), at the bases of the tentacles (cr), and in the integu- 
ment of the posterior part (cd). 
