( 373 ) 



XIX. — The Early Development of Cribrella oculata (Forbes), with Remarks on 

 Echinoderm Development. By Arthur T. Masterman, M.A., D.Sc, F.R.S.E., 

 Lecturer in Zoology in the School of Medicine of the Royal Colleges, Edinburgh. 

 (With Five Plates.) 



{From the Laboratory of the Royal College of Physicians, Edinburgh.) 

 (Read March 3, 1902. Issued separately May 27, 1902.) 



TABLE OF CONTENTS. 



Introductory ..... 

 Brooding Habits .... 

 Methods Employed 

 Periods of Development — 

 Part I. Embryonic Period 

 Segmentation 

 Gastrulation 

 Part II. Larval Period 



External Appearance 

 Internal Structure . 

 Stage A 

 „ B . . 

 „ C . . 

 ,. D . 



PAGE 



373 



374 

 375 



377 

 377 



378 

 382 

 382 

 385 

 385 

 385 

 386 

 387 



Part III. Post-Larval Period 

 Stage E . 

 F 



General Considerations . 



Comparison with other Asterids 



Relation of Larva to Adult 

 Theoretical Considerations 



The Bilateral Ancestor 



Transition to Echinoderms 



Comparison with other Groups of Echino 

 derms ..... 



Summary 



List of Illustrations 



PAGE 



388 

 389 

 389 



393 

 395 

 399 



402 

 402 

 405 



408 

 414 

 416 



The breeding habits of this asterid have been noted by M. Sars, # who has described 

 the external features of the developmental stages. He observed the peculiar habit of 

 1 brooding.' In the spring of 1840 he found several of the parents at very low-tide-mark 

 fixed to stones in their brooding position. The species he examined is described as 

 Echinaster sanguinolentus, and is apparently the same species, though he speaks of the 

 ' blood-red ' embryo, whereas my specimens were a light orange. They conform exactly 

 to the tint of the oral surface of the parent. The time of breeding of Cribrella oculata 

 at St Andrews appears to extend from February to April inclusive, and throughout this 

 period the parents are found fairly plentifully, adhering to the under surface of rocks, 

 nursing the masses of eggs or young under their bodies. This habit of protecting its 

 young is also pursued by an allied species, Asterias miilleri, which is equally abundant 

 in the same locality, but the breeding of which is effected earlier, i.e., in November and 

 December. I hope to proceed with the study of this species in the near future. 



Ovaries. — The ovaries of Cribrella oculata do not differ in essential structure from 

 those of the common Asterias rubens, though they contain fewer and larger eggs. The 



* Sars, M., Fauna Littoralis Norvegim, Erste Lieferung, Christiana, 1846, p. 47, Tab. 8, f. 1-37. 

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



