288 E. W. MACBRIDB. 



Pig. 75. — A section of a larva of' Stage G. Shows the dorsal pyloric cseca 

 and their suspensory mesenteries. 



Tig. 76. — A section of another larva of Stage G. Compare with PI. IV, 

 fig. 61, and note that the arm rudiment No. V (not marked in the figure) has 

 now become applied to the lobe No. 1 of the hydrocoele. The stone-canal is 

 seen opening into lobe No. 2, and the perihsemal rudiment 1.2 has grown out 

 into a canal insinuating itself between the ectoderm and the hydrocoele. 



Figs. 77 and 78. — Two sections of a rather older larva. Fig. 77 shows that 

 the right ventral and right dorsal horns (/'jo'c'. and l"p"d') of the left 

 posterior ccelora have coalesced, and that the left posterior coelom has thus 

 acquired a ring-like form. Pig. 78 shows the formation of the anus of the 

 adult. 



Fig. 79. — Section parallel to the adult plane of a larva of Stage F. Shows 

 tUe relationships of the axial sinus, oral ccelom, and water-vascular ring (lovr.), 

 the last being still incomplete; also four perilieemal rudiments alternating 

 with the five hydrocoele lobes. 



Figs. 80 and 81. — Two sections in same plane as Fig. 79 of a larva of Stage 

 G. Fig. 80 shows the axial sinus (a'.) in process of growth to form the inner 

 peribsemal canal. Fig. 81 shows the completion of the water-vascular ring at 

 the spot marked by the asterisk between the hydrocoele lobes Nos. 1 and 5 ; it 

 also shows the trifid form of the adult oesophagus before the mouth is formed, 

 and the oral ccelom opening into the left posterior coelom. 



Fig. 82. — Similar section of older larva in which mouth is formed. The 

 five interradial lobes of the "stomach" are present, the trifid shape having 

 disappeared; and the retractor muscles of these lobes are formed from remnants 

 of septum between oral and left posterior coelom. The distance (U) from tip 

 of arm to centre of disc "36 millimetre. 



PLATE XVI. 



Figs. 83 and 84. — Two more sections from the same series as Fig. 82. 

 Fig. 83 shows the pyloric sac with its five ceeca just beginuing to be bifid, 

 and the mutual relations of the right hydrocoele and axial sinus ; also the stone- 

 canal opening into the latter. Fig. 81 shows the point of origin of the rectum 

 and the rudiment of rectal caecum and the relation of right posterior coelom to 

 the pyloric cseca. In Fig. 83 {pr.germ.inv.) is the involution of peritoneum 

 from which the primitive germ cells are formed. 



Figs. 85 — 94 represent sections of abnormal larvse. These sections are cut 

 parallel to the larval plane, except Fig. 90, which is rather oblique to that 

 plane. Magnification the same as before. 



Figs. 85 and 86. — Two sections of a larva of Stage D, or slightly 

 younger, rhi/. Eight hydrocoele developed into two distinct lobes 

 lined with cubical epithelium. 



