THE STARFISH SOLASTER ENDECA. 17 



The characters of the larva at the end of the free-swimming stage may be summarized 

 as follows: — The preoral lobe is bent sharply to the left, and twisted dextrally* 

 through an angle of approximately 75 degrees; the arms are long, glandular at the 

 tips, muscular, and capable of anchoring the larvae temporarily by adhesion to 

 smooth surfaces ; the body has become disc-like in shape through expansion in the 

 larval sagittal plane ; four notches are beginning to appear on the dorsal and posterior 

 edges of the disc marking the position of future interradii. A much larger notch 

 bounded by prominent lips is becoming evident on the right side at the root of the 

 preoral lobe (p. 22). The first five radial pouches of the hydrocoele in some cases 

 appear externally before fixation, while in most they are not evident till the fixed 

 stage has begun. It will be more convenient to include the account of their 

 appearance under the heading of metamorphosis (p. 20). 



The above changes took place as follows : — 



6 days after fertilisation . . . Gastrulation complete. 



7 „ ,, ... Gastrula elongated to form a cone. 



8 „ „ ... Ventral dimple present. 



9 „ „ ... Indication of larval arms, and of ectodermal pit for the 



hydropore. 

 10-11 J, „ ... Arms longer and coming together so as to conceal the sucker; 



the larval body flattening in the sagittal plane ; the blasto- 

 pore closed. 

 13-18 „ „ ... Body becomes disc-shaped ; the characteristic flexion and 



torsion of the arms in progress ; larvse able to adhere by 

 the tips of the arms. 

 19 „ „ ... The majority of the larvae now attached by the sucker. 



(For earlier chronology, see p. 13, aud for later, p. 25.) 



V. General Histoey and External Characters during and after Metamorphosis. 

 (PI. I. figs. 12-14 ; PL II. figs. 22, 23.) 



Under this heading two periods have to be distinguished — the one of attachment by 

 means of the sucker whose development we have already seen in the larva (p. 14), and 

 the other during which, the sucker having atrophied, the earliest tube-feet are the 

 attaching-organs. It is during the first of these periods that the greatest changes take 

 place. The second is chiefly concerned with the acquirement by the various organs 

 of their more perfect form and of their functional activities. 



Attachment hy the Larval Sucker. — In order to attach themselves, the larvae cause 

 their arms to diverge widely, so that the sucker becomes prominent and is thus 

 enabled to adhere readily to a suitable surface. The attachment is by true sucking 

 action and not simply through the formation of an adhesive cement. At this stage 

 the sucker is oval in shape with the longer diameter transverse, and, like the tips of the 



* See note on p. 13. 



VOL. XX. — PART I. No. 3. — February, 1912. d 



