292 E. W. MACBKIDE. 



organ, nerv. larv. Larval nervous tissue. Fig. 133 is through the dorsal part 

 of preeoral lobe; Fig. 135 through its ventral tip. Magnified 480 diameters. 



Fig. 136. — Section of the adhesive disc of larva shown in Plate 21, figs. 

 62 — 69. X. A small piece of alga, to which it adheres by a secretion of 

 mucus, hist. Involutions of neighbouring portions of ectoderm to undergo 

 destruction by amoebocytes, amceh. ; by this means the prseoral lobe is reduced 

 in size. Magnified 480 diameters. 



Fig. 137. — Section of the lateral wall of prseoral lobe of larva of Stage D. 

 muse. larv. Larval muscles derived from the peritoneal cells. Magnified 1000 

 diameters. 



Fig. 138. — Section through the ectoderm and hydrocoele wall of a larva of 

 Stage D, to show the characters of the various larval epithelia. Magnified 

 1000 diameters. 



Fig. 139. — Similar section from a larva between Stages E and F. A peri- 

 hffimal rudiment is shown. Magnified 1000 diameters. 



PLATE XXI. 



Figs. 143, 149, and 150 are magnified 600 diameters, the rest 1000 dia- 

 meters (Leitz's immersion ^). 



Fig. 140.— Similar section from a larva of Stage F (that shown in Figs. 

 62 — 69). nerv. The incipient nervous tissue developing as a fine plexus 

 amidst the bases of the ectoderm cells. 



Fig. 141. — Similar section from a young star-fish, in which K, equals 

 •4 millimetre. Nerv. circ. Nervous ring. calc. Calcigenous tissue. Jibr, 

 Fibrous tissue, reir. muse. Retractor muscles of stomach. 



Fig. 142. — Developing eye of same star-fish. A simple ectodermic pit is 

 seen. 



Fig. 143. — Eye of star-fish from which Figs. 129 — 132 are taken, ret. 

 Visual cells, vit. Cells functioning as " Glaskorper." 

 Figs. 144 — 148 illustrate the differentiation of tissues in the body-wall. 

 Fig. 144. From the right side of a larva of Stage D. At * a cell is 

 seen in the act of dividing, to form one of the amoebocytes of the 

 ccelom. 

 Fig. 145. From larva of Stage E (that shown in Figs. 51 — 53). gob. 

 Goblet cells, muse. Developing muscles ; as yet they are simply tails 

 of the coelomic epithelium. Jibr. First rudiment of the fibrous tissue. 

 Fig. 146. From the young star-fish from which Fig. 141 is taken, calc. 



Small portion of calcigenous tissue. 

 Fig. 147. From the young star-fish from which Fig. 143 is taken, and also 



Figs. 129—132. 

 Fig. 148. Ectoderm of another specimen of same age, to show the sense- 

 cells. 



