510 CHARLES W. HARGITT 
6. The larva, planula. The life history of the morula is com- 
paratively brief, perhaps from six to eight or ten hours, the 
period varying considerably. During this time the definitive 
ectoderm has been established, cilia developed and the free-swim- 
ming larva, the planula, begins its career. Concerning the 
structure of this organism there is no occasion for special details. 
It differs little if at all from that characteristic of others whose 
structure has: been repeatedly portrayed, and is too well known 
to need further account. In the present instance, as in those 
of numerous others, at the time of the assumption of this condi- 
tion the larva is still a solid mass, with little organization beyond 
the above mentioned ectedermal differentiation. A definitive 
entoderm may not become established till relatively late in larval 
life, as I have repeatedly pointed out in other cases, and only 
after a process of physiological differentiation, as shown in a 
later section. The first evidence of a coelenteron appears as a 
slit-like cavity in the larval axis, which later enlarges as the reduc- 
tion and absorptionof the pro-entodermic mass proceeds. Finally, 
by such graduated method does the entoderm become estab- 
lished. At no time is there a mouth or other means of communi- 
cation with the outside during phases of embryonic or larval his- 
tory. Planulae of Hydractinia have been frequently reared under 
artificial conditions, and readily transform into the final, or polyp 
state. Soon after the larva attaches itself the mouth is established 
by a terminal opening which arises by a rupture and rearrange- 
ment of the adjacent cells. Tentacles arise in the usual manner, 
first three in number, followed shortly by three others at inter- 
mediate points, and slightly below the first series. At the base 
of the polyp there arise root-like stolons, two or more in number, 
which mark the origin of the hydrorhizal network characteris- 
tic of the species. 
C. Clava leptostyla Ag. 
In connection with the work on Hydractinia I have taken occa- 
sion to re-examine the material upon which was based the work 
embodied in my previous paper on Clava (06), and have also care- 
fully studied sections of new material which had been fixed in 
