Development of Holothnria Floridana Pourtales 215 



By the time of hatching, the Polian vesicle has arisen from the 

 circular canal in the left ventral interradins. It is well marked 

 in the seventh day, and by the ninth, extends posteriorly one-half 

 the length of the ccelom. Joh. Miihler, 1852, in the "Auricularia 

 mit Kugeln" and Ludwig, 1891, in Cucumaria planci, describe the 

 origin of the Polian vesicle in the left dorsal interradins. However, 

 Thompson, 1862, in Synapta inhserens, states that the Polian vesicle 

 arises in the left ventral interradins, as I have found it in Holothnria 

 floridana. 



The Polian vesicle enlarges rapidly and nearly fills the left half 

 of the ccelom. By the sixty-seventh day it extends three-fourths 

 of the way to the posterior end of the body. During this time the 

 anlagen of the radialia and interradialia of the calcareous ring are 

 being established. 



From observation of the living holothurids it is apparent that 

 the respiratory movements of the cloaca begin quite early in the free 

 larvae. However, sections of my stages do not clearly reveal the 

 presence of respiratory trees until the fortieth day, although at that 

 time they are well developed, the left being the larger. Hence, it is 

 probable that they arise in an earlier stage. In some cases only 

 one resj)iratory tree is to be found. The radiating cloacal dilator 

 muscles, which cause the respiratory movements of the cloaca, are 

 also well established at an early date. 



On the third day, the anlagen of the first calcareous spicules ap- 

 pear. Beginning with a simple short rod, the ends bifurcate to form 

 a four-rayed base. Sometimes only three rays are developed. The 

 outer ends of the rays fork. From the central part of the bar arise 

 the four vertical rods which are joined together by cross-beams to 

 form the spire. The ends of the branches of the rays again fork, and 

 the apposed terminal branches from neighboring rays gTow together 

 to form the four central holes of the disk. Then the circle of 

 peripheral holes is developed in the same manner. The rosettes and 

 perforated plates are formed just as the disks of the tables. 



In the tentacles, along with the tables and perforated plates, by 

 the fourteenth day, circular and spiral supporting rods are developed. 

 The spiral rods extend from three-fourths to one, two, or even three 



