58 DEVELOPMEA'T OP THE STAEFISH SOLASTER ENDECA. 



PLATE I. Figs. 1-14. 



(For lettering see pp. 55-56.) 



Figs. 1-4. Stages in segmentation. (1-3, X 20 ; 4, X 25.) In tig. 4 slit-like spaces 



are beginning to appear in the cell-mass. 

 Fig. 5. Section of advanced stage in formation of blastula by egression of central cells 

 along many lines (p. 12). The flattening on the underside is due to 

 commencing gastrulation. ( X 30.) 

 Fig. 6. Vertical section of the fully-formed gastrula (p. 12). (x 30.) 

 Fig. 7. Diagram from right (aboral) side of larva at end of free-swimming stage, to 

 illustrate the early relations of the hydroccele pouches to the aboral arm- 

 rudiments (see p. 22). The pouches are lettered with romau numerals, 

 and the aboral arm-rudiments, with which these finally become associated 

 to form the rays of the Starfish, are indicated by corresponding arabic 

 numerals, (x 20.) 

 Fig. 8. Larva towards end of free-swimming stage cleared and looked at from right 

 side, showing the developing skeletal elements. The spines (sp.) are 

 represented by numerous small nodules all over the field (p. 43). Of the 

 aboral plates, six pairs of terminals (t.) have appeared, as well as a number 

 of the plates {r.pl.) which develop over the epigastric area (p. 41). Seen 

 more faintly as at a deeper level and partly out of focus are five pairs of 

 ambulacrals {am.) (p. 41). (x 45.) 

 Fig. 9. A few spines and plates of the aboral skeleton as seen in a specimen twelve 



weeks old (p. 42). (x 200.) 

 Fig. 10. Microphotograph of sagittal section of advanced larva, showing particularly 

 the statolith-like body on the posterior coelom (p. 46). The high 

 epithelium over the crown of the preoral lobe will also be seen (p. 46, 

 nervous system), (x 30.) 

 Fig. 11. Diagram from aboral side, illustrating the relations of the radial and rectal 

 caeca, the stomach, and the anus in a specimen four and a half months 

 old. Description on p. 40. (X 35.) 

 Fig. 12. Small portion of disc from aboral side in a specimen of about same age as in 

 fig. 13, but more highly magnified. The papular appearance due to the 

 developing spines is indicated, the latter being outlined as in a cleared 

 specimen. (X 45.) 

 Fig. 13. Young Solaster at time of formation of mouth. The triradiate shape of the 

 mouth will be noted (p. 23). The ninth radial pouch of the hydroccele 

 has just appeared. The arm-rudiments for arms VII-IX have not yet 

 separated from one another. The sucker is now small and outside of 

 the hydroccele-ring (p. 21). (x 45.) 

 Fig. 14. Young Solaster about six months old, from oral side, showing charac- 

 teristic appearance of spines, ambulacral grooves, denticles, etc. ( X 45.) 



