EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 389 



Stage E is shown in figs. 77 and 78, a view from the right and left side 

 respectively. The body is seen to be expanded in a vertical plane and correspondingly 

 flattened laterally. The expansion is most evident dorso-ventrally, converting the body 

 of the larva into a disc. The pre-oral lobe is not so prominent, and the median process 

 is more widely separated from the ventro-lateral processes. An anterior view (fig. 79) 

 shows the formation of a sucker between the three processes, on the ventral surface of 

 the pre-oral lobe. This sucker is used for attaching the larva as described, a mode of 

 fixation of a different nature from the temporary habit indulged in by the larva of 

 stage D. In the latter case the larva appears to adhere by the tips of the three pre- 

 oral processes. 



This figure indicates the commencing torsion of the body or disc upon the stalk or 

 pre-oral lobe. This torsion will be alluded to later. 



In fig. 78 the first indication of axial symmetry is seen in the five-lobed condition of 

 the hydroccele. 



Stage F — (figs. 105-106) — shows considerable progress. The destruction of the 

 larva is indicated by the degenerating pre-oral lobe, though the sucker is further 

 elaborated and the median process is very commonly split into two. Progress in the 

 young starfish is exhibited by the rapidly developing groups of tube-feet, whilst 

 dorsally the adult body is beginning to show a five-lobed condition. The five radii of 

 the hydroccele are here numbered in the order of their usual development, namely, from 

 the 'mouth-region' counter-clock- wise. Thence the antero-ventral is 1, the postero- 

 ventral is 2, the posterior is 3, the postero-dorsal is 4, and the antero-dorsal is 5. 



We may now notice the internal changes from D to F inclusive. The transverse 

 larval series (figs. 71-76), is through a very important stage, D/E, illustrating the 

 commencement of progress to the adult organisation. Compared with the similar 

 series of stage D, there does not at first sight appear to be much difference, but 

 we can see (in fig. 71) that anteriorly the termination of the ventral horn of the 

 left posterior ccelom is expanding vertically and horizontally into two prominent 

 processes ; the horizontal process passes under the pre-oral ccelom over to the right- 

 hand side, whilst the vertical process hangs downwards towards the ventral surface of 

 the larva. On the left of the figure the left lateral ccelom has thrown out two processes 

 anteriorly, which are the first and fifth radii. An inspection of the remaining figures 

 shows the second, third and fourth, all well marked. The left lateral ccelom is therefore 

 now axially symmetric, forming a star with five blunt processes. We may now call it 

 the hydroccele. The water-pore now opens to the exterior in the middle of the right 

 side, between figs. 73 and 74. Lastly, the two posterior sections, i.e. figs. 75 and 76, 

 show the right and left posterior cceloms fusing with one another. This fusion first 

 takes place at the distal end of the right element, and gradually the double wall becomes 

 thinned out backwards till the two are merged into one cavity. In fig. 76 the remaining 

 septum is still intact posteriorly, though in fig. 75 it has broken down further forward. 

 This fusion of right and left posterior ccelom is extremely important from the point of 

 TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 19). 3 n 



