1918 Osferud: on Embryology of Leptasterias 3 



Gradually these arms lift from the surface, grow rapidly, and markedly 

 increase the size of the embryo, bursting the enclosing membrane (Figs. 

 1, 5, 6). The larval arms are at first immotile and nicely symmetrical; 



1 median, ventral, and unpaired; the other 2 forming a dorsal pair. V^ery 

 early in their development the tips of the larval arms become adhesive 

 knobs, enabling the larvae to attach themselves to each other and to the 

 mother. At the upper pole of the preoral lobe between the larval arms 

 an apical elevation is produced, homologous with the fixing disk of Asterina. 

 This likewise forms adhesive cells, develops slowly into a terminal knob, 

 and finally becomes the sole larval organ of attachment. The perfect 

 bilateral symmetry which at first obtains is soon disturbed when the mus- 

 cular development of the larval arms permits their movement. The larvae 

 then become very active and the distortion of the preoral lobe may become 

 very pronounced ; so much so that it becomes difficult to recognize which 

 of the arms is which and consequently which is right or left side. No 

 larval mouth is ever developed. This feature as well as the genera] forui 

 remind one of the larvae of Cribrella and Solaster. 



The first indication of the approaching metamorphosis is given by 

 the appearance of a flattened disk. This arises on the left side of the larva. 

 It is not situated exactly laterally but somewhat asymmetrically, occupying 

 an obliquely latero-ventral position (Fig. 7). This disk is at first small, 

 but as it grows it spreads out over the body of the larva. At its margin 

 5 lobes of the hydrocoele become superficially evident (Fig. 8). In a 

 sinistral view these 5 lobes or pouches may be numbered consecutively in 

 a clockwise direction as indicated in Figure 8. It is then seen that pouches 

 i and 5 are separated by a wide interval occupied by the broadly attached 

 preoral lobe. The body of the larva soon begins to show positive signs 

 of transformation into the body of the future starfish. It becomes com- 

 pressed from left to right side and expanded correspondingly in other 

 directions, revealing what are to become the oral and aboral sides of the 

 starfish. At the margins of this compressed body outgrowths appear 

 situated aboral to the hydrocoele pouches. These outgrowths are the 

 apices of the starfish rays of which only 5 are at first distinctly visible. 

 From figures 10 and 11 it will be seen that a plane passed horizontally 

 through the body fundament of the starfish does not correspond to a sagittal 

 plane of the larva; but looking at the larvae in the direction of its postero- 

 anterior axis, there is an anti-clockwise torsion of the body of the larva 

 on the preoral lobe. 



Before any indication of a (ith hydrocoele pouch apjjcars, the first 

 5 pouches develop fundaments of the tube-feet (Fig. 12). By the time 



2 pairs of tube-feet are distinctly developed in each of the T) rays, the 

 first superficially visible rudiment of the 6th hydrocoele pouch comes 



