554 D[l. J. F. GEMMILL ON THE 



In aecounts of the development of Asterias ruhens (2, p. 233) 

 and Porcmla (3, p. 33) I put forward the view that the mass in 

 question is the rudiment of a posterior enteroccelic outgrowtli, 

 pointing out that the recognition of potential metamerism in 

 echinoderm development makes for simplicity and for the recon- 

 ciliation of what might otherwise seem fundamental differences 

 in the ontogeny of different forms. (See also p. 557.) 



The larvae above desci-ibed add evidence in support of this 

 view. In any event, they deserve to be put on record since they 

 supply a definite example of a developmental variation occurring 

 along lines which, if persisted in, could give rise, in course of 

 time, to veiy important alterations in the ontogeny, without 

 necessarily affecting the adult anatomy, of the species concerned. 



II. Oribrella oculata. (PI. II. figs. 6-10.) 



Early in April 1915 a number of Gribrell^c, obtained at low- 

 water on the shore near the Millport Marine Station, were placed 

 in the aquarium tanks, where they spawned freely after a few 

 days. The eggs, which were kindly put at my disposal by the 

 Superintendent, underwent natural fertilization, sets of them 

 being brought through metamorphosis both at the Marine 

 Station and in the Embryological Laboratory at Glasgow 

 University. 



Masterman in 1902 (9) furnished an excellent account of the 

 development of this species. My observations confirm liis results 

 in most respects, but I am able to give new or supplementary 

 data on various points *. 



(1) Spawning. — jSTot less (aiid probably more) than 500 eggs 

 are produced by each full-sized female. None of the eggs was 

 observed to enter into and remain within a brood-pouch formed by 

 the closure of the arms ai'ound the mouth. In my specimens the 

 genital openings look outwards in the interradii. During March 

 and April Oribrella eggs or larvae are not infrequent in the 

 plankton of the Firth of Clyde. 



Sars (10, p. 170) and Masterman (9, p. 374) have described the 

 brooding habits of Oribrella, the latter pointing out that these 

 are by no means intense, but adding that it is not known 

 whether in natural conditions any of the larv* leave the brood- 

 chamber of the mother. He gives the number of eggs laid by 

 each female as 30 to 50, and Sars as 10 to 30. Masterman also 

 states (9, p. 374) that the genital openings are situnted on the 

 oral surface of the starfish at the edge of the oral disc, this being 

 an adaptation for brooding. 



Oribrella appears to be a species in Avhich the brooding habit is 

 either being acquired or being lost. The second alternative seems 

 rather more probable, and from the evidence given above it would 



* Dr. Masterman makes the very reasonable suggestion to me that the features 

 noted under 1 and 2 below are probably explicable as adaptations to development 

 within a relatively enclosed sea-area, such as is formed by the Firth of Clyde. 



