THE STAEFISH SOLASTEE ENDECA. 19 



disappearance of the whole preoral lobe except the sucker, and, finally, by atrophy 

 of the sucker itself. 



The points mentioned above will now be gone over in more detail, the first three 

 being taken together. 



1, 2, 3. Flexion &g. of Preoral Lohe, and Formation of the Oral and Ahoral 

 Surfaces. — The commencement of the flexion and torsion of the preoral lobe was 

 noted before the end of the free-swimming stage (p. 16). 



The preoral lobe became sharply bent at the neck towards the left side of the body, 

 and at the same time twisted in the dextral direction as seen from the anterior end. 

 Roughly speaking, the combined movement may be compared with that which would 

 result in the human subject from extreme overaction of the left sterno-mastoid muscle, 

 fhe head representing the preoral lobe of the larva. The back of the preoral lobe is 

 brought to begin with obliquely against the left body- wall of the larva. This is the 

 condition at the end of the free-swimming stage. Later the movements are carried 

 out to their fullest degree, the leftward flexion being nearly through the full half-circle 

 of 180 degrees, while the torsion is through a right angle. At the same time, the back 

 of the preoral lobe is taken into or incorporated with the left body-wall of the larva. 

 As a result, the anterior or frontal larval arm comes to look outwards from near the 

 middle of the left side of the larva, or, rather, of the oral surface of the Starflsh, as this 

 side may now be called. The sucker and the paired larval arms also look outwards 

 from the oral surface, but are nearer that part of the margin which in the larva was at 

 the junction between the preoral lobe and the larval body. 



The changes above described are illustrated in figs. 21-23 of PI. II. As they 

 proceed, the preoral lobe settles down more and more on the oral surface, from which 

 it finally ceases to project. The smoothing out of its walls involves a considerable 

 addition to the central area on the oral surface. This is needed in connection with 

 the very marked widening of the hydrocoele ring that is also taking place, and it 

 provides a sufficient space, in the middle of which the mouth-opening can afterwards 

 break through. 



It will be remembered that the lower pole of the gastrula (posterior end of the 

 larva) was paler in colour than the upper pole (anterior end of larva). The changes 

 described above result in the massing of the richly pigmented epiderm (anterior end 

 epiderm) into the area on the oral side of the Starfish which overlies the circle of the 

 hydrocoele. When the radial canals of the hydrocoele grow out they are accompanied 

 on the surface by corresponding extensions of pigmented epiderm, which are broad 

 enough also to overlie the sucker-feet when these are formed in their turn. The purely 

 actinal surface of the young Starfish is thus derived chiefly from the body-wall of the 

 anterior end of the larva. Conversely, we find that the aboral surface is now covered 

 with the pale epiderm which in the perfectly bilateral condition was confined to the 

 posterior part of the larva. It is only in the region of the notch opposite the larval 



d2 



