16 DE. J. F. GEMMILL ON THE DEVELOPMENT OF 



glandular ridges at the tips of the arms. During life, the action of the cilia can 

 readily be observed under the microscope by keeping the larvae on the stage in a 

 suitable cell and using strong oblique illumination. While ciliation is fairly uniform 

 all over, it vpas noted during the first week that the cilia which covered the anterior 

 pole seemed larger than the average. They did not, however, form a definite tuft 

 standing out from the rest. The strongest and most persistent currents produced by 

 the cilia pass from anterior to posterior end of the larva all along its sides, and it is 

 this ciliary action which causes the forward progression of the larva. More than once 

 while I was watching this action under the microscope, the direction of movement 

 quickly became reversed ; streams of particles began to flow over the anterior end, and 

 had the larva been free to swim about, it would undoubtedly have moved with the 

 posterior end in advance. Reversals of this kind occurred with relatively greater 

 frequency in the specimens I was working with on the stage of the microscope than 

 in those which remained undisturbed in the aquaria. 



At the posterior end of the early larva, external to the blastopore, there is a narrow 

 annular area of cilia which lash in an outward direction. The current thus produced, 

 taken along with the stronger currents passing backwards over the general surface of 

 the body, results in a forward eddy of small particles towards the blastoporic opening. 

 After the blastopore has closed, the annulus in question becomes uniform with the 

 rest of the surface of the body. 



It was stated above that the formation of the hydroporic opening on the right side 

 is the first interference with the external bilateral symmetry of the larva. The next 

 departure from this symmetry is a more obvious one, and results in a change iu the 

 relative position of the preoral lobe and the body of the larva (PI. II. figs. 19, 20). 

 Two movements are involved, namely, (1) lateral flexion of the preoral lobe towards 

 the left side of the body, (2) torsion of the neck of this lobe in the dextral * direction 

 as the larva is looked at from the anterior end. Both movements occur together and 

 in a gradual manner, and both are completed only two or three days after fixation by 

 the sucker has taken place. Meantime their result is to bring the back of the preoral 

 lobe obliquely against the left side of the body, a very sharp angle being formed on 

 this side in the region of the neck (fig. 20). The whole movement is of greater 

 rapidity and acuteness than can be accounted for simply in terms of differential 

 growth, and as a matter of fact it seems to be caused by the action of the special 

 band of muscular fibrillas (PI. II. fig. 19 ; PL III. fig. 34) which is referred to further 

 on (p. 47). 



Now commences a process of much interest, whereby the walls of the preoral lobe 

 are in part incorporated with the left body-wall of the larva (PI. II. figs. 22, 23). But 

 this takes place during metamorphosis and will be described later, along with the 

 completion of the flexion and torsion of the preoral lobe (pp. 19, 27). 



* See note on p. 13. 



