THE DEVELOPMENT OF OPHTOTHHIX ERAGILIS. 605 



the opening of tlie stone-canal into the left hydrocoele between lobes 1 and 2. 

 Fig. 42 b shows the opening of the pore-canal to the exterior, m.p. Madre- 

 poric pore. st. c. Stone-canal. X 260. 



Tigs. 43 a and b. — Two longitudinal frontal sections through a larva picked 

 out of Plankton, in wliicli tiiere is ihe first discoverable trace of metamorphosis. 

 Fig. 43 a is the more dorsal section, and passes through lobes 1 and 5 of the 

 left liydrocfEJe, whilst fig. 43 b passes through lobes 2, 3, and 4, thus showing 

 that the hydrocoele is commencing to encircle the oesophagus. In fig. 43 a 

 the right hydrocoele (r. //^.) is grazed, and the thickening of the outer wall of 

 the left posterior coelom, which is the first rudiment of an adult arm, has made 

 its appearance. The prolongation of the lumen of the coelom into the thicken- 

 ing is the origin of the dorsal coeloniic canal of tlie arm. Pari passu with the 

 extension of the left hydrocoele goes the extension of the left half of the adoral 

 ciliated band {cil. ad.). X 2C0. 



Figs. 44 a, b, and c. — Three longitudinal frontal sections through a larva 

 in the first stage of metamorphosis. Fig. 44 « is the most dorsal section, 

 fig. 44 c the most ventral. Fig. 44« shows the pore-canal {p.c.) opening 

 into the left anterior coelom (/. a. c) from which is given off the perihsemal 

 rudiment {ph. 1, 2). Both left dorsal (/". ;/'. c".) and right ventral (/'.;/. c'). 

 horns of the left posterior cfiblom are seen. From the former is given off the 

 periluiemal rudiment {ph. 2, 3), from the latter the perihsemal rudiment 

 {ph. 3, 4). The right iiydrocoele is seen, as are also the adult arms 1 and 

 II, which now show as external protuberances. In fig. 44 A all five lobes of 

 the left hydrocoele are shown. The stomodaeum, or thin-walled portion of the 

 oesophagus, is pushed over to the right, owing to the enormous growth of the 

 adoral ciliated band. p. d. Left postero-dorsal larval arm carried forward by 

 the preponderant growth of the left side. In fig. 44 c the tips of the lobes of 

 the left hydrocoele are seen projecting into the stomodaeum. Each lobe is 

 covered with an ectodermal thickening. The left antero-lateral arm has passed 

 over towards the right, so as to assume a mid-dorsal position. The opening 

 of the stomach into the intestine is shown, the different histological characters 

 of these two portions of the alimentary canal being strongly marked, x 250. 



PLATE 36. 

 Fig. 45. — Longitudinal frontal section through a larva of about the same 

 age as that shown in fig. 44. Picked out of Plankton. The rotation of the 

 left antero-lateral larval arm has not proceeded so far as in fig. 44. All five 

 lobes of the left hydrocoele are shown. Three perihaemal rudiments are seen ; 

 one of them {ph. 3, 4) appears as a hollow outgrowth of the left posterior 

 coelom. The stomach {si!) is seen opening into the intestine. External to 

 the right posterior coelom a mass of mesenchyme cells is seen — probably the 

 rudiment of one of the adult plates. X 250. 



