400 W. K. BROOKS ON THE LIFE -HISTORY 



to temperature, while Donaldson's experiments show that it is not the effect of light. 

 There is no evidence to show that it is due in any way to the direct influence of sur- 

 rounding conditions, and I think we must believe that it has been established in each 

 species by natural selection on account of some advantage to the animals which exhibit 

 it. 



The fact that the hour for discharging the reproductive elements is, in so many 

 species, a definite one, often in the night-time, shows the importance of marine observa- 

 tories where the naturalist may keep his specimens under his eye at all hours of the day 

 and night; for, as midnight collecting is usually impracticable, the early stages of many 

 animals cannot be procured without facilities of this sort. 



The eggs of Eutima mira develop rapidly, and the swimming planula stage is 

 reached early in the morning after the eggs are laid. 



Segmentation is total, but as shown in PI. 38, fig. 1, it is not perfectly regular. A 

 capacious segmentation cavity, fig. 8, a, soon makes its appearance, and the cells which 

 are a little larger at one pole than they arc at the other arrange themselves in a single 

 layer, b, and continuing to subdivide soon become nearly uniform in size as shown in 

 fig. 3. The embryo now becomes ciliated and, rising from the bottom, assumes the well- 

 known pear-shaped outline of the hydroid planula, fig. 4, with a spacious segmentation 

 cavity, surrounded by a single layer of ciliated cells, b, which are much thicker at t the 

 small end of the pear than over the rest of the body. While the blastoderm consists of 

 only one layer of cells, the planula increases considerably in size, and appears to have 

 some method of nourishing itself. 



According to Merejkowsky (50) the central cavity of the planula of Obelia com- 

 municates with the exterior through a great number of minute pores which are situated 

 between the blastoderm cells. He says the pores are large enough to permit small in- 

 fusoria to pass through them into the central cavity where he has seen small animals swim- 

 ming actively. He believes that these small organisms serve as food, and, although I 

 have not been able to discover the pores in Eutima, I have satisfied myself that the plan- 

 ula does obtain food in some way and increases in size. 



The endoderm cells soon begin to make their appearance at the small or posterior end 

 of the cavity and are set free, as shown at e, in fig. 4. They soon arrange themselves in a 

 continuous layer or endoderm over the whole inner surface as shown at e, in fig. G. 

 According to Merejkowsky, they are not formed by the transverse division or delamina- 

 tion of the blastoderm cells, but by migration, in the manner which has been described 

 by Schultze, Metschnikoff and others in the sponge planula. In a preliminary paper 

 on the life-history of Eutima (11) I have stated that they are formed by delamination, 

 but as I made no attempt to watch the changes of a single cell, I did not actually wit- 

 ness the process of division and it is possible that they are not formed in this way but 

 by migration. The formation of the endoderm cells goes on rapidly and the planula 

 soon appears to become a solid mass of cells, fig. 5, but careful examination will show 

 that a small central digestive cavity, fig. 5, g, persists in the axis of the embryo, 

 although it is rendered almost invisible in the living planula by the increasing capacity 

 of the endoderm cells, which are apparently distended, so that they almost meet in the 

 centre. In a specimen which has been killed with osmic acid and stained with picro- 



