29S UVLSOX. [Vol. IX. 



found with a little mass of protoplasm and a nucleus sticking 

 to one side, but I could come to no conclusion as to their mode 

 of formation. The bow-shaped spicules, mentioned as present 

 in the earlier stage, are now found in greater number, but still 

 there are only a few of them. In the hollow of the bow there 

 is an accumulation of protoplasm with a nucleus, and the indi- 

 cations are that the spicule is formed as a superficial secretion 

 of this mass of protoplasm. The bow-shaped spicules are almost 

 all found in the posterior half of the larva. The same is true 

 of the third kind of spicule, the rosettes of embryonic shovels. 

 No rosettes are shown in Fig. 29, but in Fig. 30 there are 

 three shown, and PL XVII, Fig. 34, represents such a rosette 

 (seen in section) more highly magnified. The spicules are 

 very thin and delicate as well as small, and are not (at this 

 time) found separately, but always united in rosettes. The 

 rosettes are few in number and are usually found close under 

 the ectoderm at the posterior pole. 



4. Metamorphosis. 



The ciliated larva swims freely for a day or two. As a rule, 

 some time during the second day after birth, it sinks to the 

 bottom and begins to attach. The first step in the metamor- 

 phosis takes place while the larva is still swimming freely 

 about. This consists in the flattening of the ectoderm. PI. XVI, 

 Fig. 27, represents a surface view of a larva 16 hours after 

 birth, and Fig. 30 a longitudinal section of the same stage. 

 On comparing these figures with the corresponding figures 

 made from a larva just hatched (Figs. 26 and 29), it will be 

 seen that the posterior unpigmented area, or region of flat 

 ectoderm cells, has increased in extent at the expense of the 

 pigmented area or region of columnar cells. By keeping the 

 same larva under observation, it can be seen that the unpig- 

 mented area gradually extends forwards. As I have said, the 

 process begins while the larva is swimming freely about. It 

 continues after the larva has sunk to the bottom. PI. XVI, Fig. 

 28, shows a surface view of a larva in course of attachment. 

 In this larva the pigmented region is reduced to a small area at 



