416 DR ARTHUR T. MASTERMAN ON THE 



3. Stage C. — Development of pre-oral lobe with dorsal and ventral processes. 

 Forward growth of dorsal and ventral horns of left posterior coelom, and backward growth 

 of left and right lateral cceloms. 



4. Stage D. — Culminating stages of bilateral larva. Dorsal process partially and 

 ventral completely divided into two. Occasional fixation. Development of pore-canal. 

 Forward growth of right posterior ccelom. Enlargement of sinistral elements. 



Post- larval Period. 



1. Stage E. — Fixation. Fusion of right and left posterior cceloms to form hypo- 

 gastric ccelom ; right lateral coelom becomes a disc-shaped epigastric ccelom ; water-pore 

 opens on right side (aboral surface) ; central ccelom becomes ' dorsal sac ' ; left lateral 

 ccelom becomes hydroccele and gives off five radii, developing in order 1, 2, 3, 4, 5 ; 

 1 and 2 ventral, 3 posterior, and 4 and 5 dorsal. Hypogastric ccelom forms aboral radii 

 over 1 and 2, partially over 3 ; opening of dorsal horn of hypogastric ccelom into 

 anterior coelom. 



2. Stage F. — Degeneration of pre-oral lobe by infoldings. Further development of 

 hydroccele into ring radii with one pair of tube-feet, often two ; hypogastric ccelom forms 

 aboral radius 4, and tip of ventral horn progresses to base of 5 ; anterior ccelom forms 

 pre-oral ccelom (in pre-oral lobe) and axial sinus ; perihsemal elements arise in inter-radii, 

 4/5 from axial sinus, the remainder from hypogastric coelom ; three (or four) elements 

 (|-, § and f ) of the oral coelom appear from hypogastric ccelom in numerical order. 



Adolescent Period. 



Stage G. — Further development of the star. Changes to be followed in later 

 paper. 



Relationship of larva to adult. 



Larval sagittal plane corresponds to discal plane of starfish. 



Left side of larva = oral surface of starfish. 



Right „ „ „ = aboral „ „ „ 



Coronal longitudinal plane of larva is at 72° to plane of adult through madreporic 

 inter-radius. In development of hydroccele and hypogastric coelom there is no torsion, 

 but, owing to extension of ventral horn only of hypogastric coelom, there is a union of I. 

 with 5, II. with 1, III. with 2, and so on. 



General considerations. 



Bilateral ancestor closely similar to Balanoglossus. Subjected to period of creeping 

 life on right side, hence sinistral asymmetry. Followed by fixed period, hence axial 

 symmetry. 





