450 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



FREE-SWIMMING LAKVA. 



Five days after the first cleavage the embryo ruptures the inclosing membrane 

 and escapes as a free-swimming larva. The length of this free-swimming life 

 varies extraordinarily, even in the different individuals of the same brood; some 

 attach themselves after a few hours and immediately proceed to further develop- 

 ment, while others, somewhat larger though with only insignificantly altered 

 structure, are to be met with still free swimming after a lapse of as much as 

 108 hours. Even at 60 hours the larvas which immediately became fixed have the 

 body divided into stalk and calyx, and possess 15 tentacles projecting into the 

 closed vestibule. 



When just liberated the larvae are almost invariably about 0.40 mm. long. 

 If they remain free swimming for a considerable time they may reach at the 

 end of the second or third day a length of 0.45 mm. to 0.48 mm., or in some cases 

 even 0.50 mm. 



The larvae usually swim with the anterior pole forward; only exceptionally, 

 as for instance when they strike an obstacle, is the direction reversed. Occasionally 

 a rotation about the major axis may be observed. 



Ectoderm. 



During the later embryonal stages the ectoderm has undergone important 

 changes, and is quite different from that of the 75 to 80 hour stage. 



Anteriorly the entire ventral part is deeply sunken so that, with its elongate 

 form, the larva now appears distinctly bean-shaped. 



The five ciliated bands are sharply differentiated from the much broadened 

 intermediate zones, and over the deepened apical depression a large tuft of cilia 

 has formed. 



The attachment pit has also deepened, and appears much more sharply differ- 

 entiated from the neighboring regions. 



Ectodermal epithelium, ciliated bands, and intermediate areas. The course of 

 the ciliated bands is in general as in the younger stages. 



The foremost band surrounds the apical pit with its tuft of cilia. On the 

 ventral side it is interrupted by the more or less circular attachment pit into 

 the ciliated borders of which its ends pass over. 



The second bow is entire; in the median line ventrally it is arched forward 

 slightly and passes between the attachment groove and the vestibular invagination. 

 Dorsally in the median plane it is arched forward somewhat, though not so much 

 as on the ventral side. Its course varies more or less in details in different 

 individuals. 



The third band is far displaced posteriorly in the median line ventrally by 

 the extension of the vestibular groove, about the posterior angle of which it runs. 

 Later, as the vestibular groove is further extended posteriorly, it cuts across this 

 third band, which sinks into it and takes part in its formation. This third band, 

 thanks to the broad ventral posterior sweep and to its slightly diagonal course 

 along the sides of the embryo, is the longest of all. 



