THE STAEFISH SOLASTEE ENDECA. 15 



Shortly afterwards the blastopore closes, and the larval body becomes marked off from 

 the preoral lobe by a groove which passes transversely across the back of what may be 

 called the neck of the larva. This groove is produced in part by dorsal flexion of the 

 head on the body, and partly through further expansion of the body in the sagittal 

 plane. The hydroporic opening on the right side is the first, and for a time the only, 

 departure from the external bilateral symmetry of the larva. 



The arms are hollow outgrowths and contain prolongations from the anterior ccelom. 

 They are at first short, blunt, and ciliated uniformly with the rest of the body. Later 

 they lengthen and acquire small glandular ridges at the tips, over which the cilia 

 become reduced. They are then able, acting either singly or together, to cause the 

 larva to adhere lightly to smooth surfaces. They have also become muscular, so that 

 they can perform movements of (1) simple shortening or retraction and lengthening 

 or protrusion, (2) spreading apart so as to expose the sucker, (3) closing together so 

 as to hide the sucker (PI. II. figs. 18, 19). The cilia are now reduced over the glandular 

 ridges, though still present in the intei'vening furrows. 



The sucker develops as a round, slightly elevated disc of much elongated cells in 

 the bottom of the concavity between the larval arms. The cells are ciliated at first, 

 but afterwards lose their cilia. 



While the larvae are elongating and becoming cone-like in shape, they remain 

 upright in the water, performing circular movements exactly like those of the gastrulse, 

 though the direction of movement was now sinistral * as seen from above in the 

 majority examined. 



After they have reached full length, one begins to find them from time to time iu 

 an oblique position in the water, the anterior end, however, being still always higher 

 than the posterior. In this position they are rotating round their own antero- 

 posterior axis as before, and in addition they are progressing slowly along a straight 

 line in the direction towards which the anterior end points. Reversal of this latter 

 movement can occur, especially under stimulation, but it is rare and lasts only for a 

 short time. 



During the first two or three days of the free-swimming stage the larvae are rather 

 lighter than the sea-water and tend to remain without effort near the surface. Later 

 their specific gravity gradually comes to equal and finally to exceed slightly that of the 

 water. When this stage has been reached the movement of progression with the 

 anterior end first will keep them from sinking helplessly to the bottom, while 

 reversal will ensure that they need not remain continually at the surface. The 

 characteristic position of the larva in the water indicates that the original difference 

 in weight between the anterior and posterior ends of the larva has been retained in 

 some degree throughout the whole free-swimming stage. 



The whole surface of the larva is richly ciliated, excepting the sucker and the 



* See note on p. 13. 



