558 DR. J. F. GEMMILL ON THE 



stage a gap oi' wide separation (aboral braclnolarian notch) 

 between arm-rudiments I. and II. of the disc. Arm-rudiments 

 I. and V. are, however, kept ajjart for a time during the retraction 

 of the preoral lobe towards the oral side of the disc, through 

 having between them that region of the preoral lobe which carries 

 the right latei'al brachium. 



A hydrocoelic (oral brachiolarian) notch between pouches I. and 

 Y. is characteristic (a) of feeding attaching bi'achiolai-ian larva?, 

 e. g., Asterias mhens (2), Asterias jxdlida (5), Forania (3) ; (6) of 

 forms proximately derived from these, e. g., Aster ina {1); (c) of 

 multiradiate forms in which the exti^a rays are added to one or 

 both ends of an open hydrocoele crescent, e. g., Solaster endeca (1) 

 and S. papposus. On the other hand, absence of a marked hydro- 

 ccelic notch is characteristic of {a) non-attaching feeding bipin- 

 nai'ijB of the Bipinnaria asterigera type ; (&) quinque-radiate forms 

 with abbreviated ontogeny, e. g., Crihrella and Asterias millleri. 

 The presence or absence of a well-marked aboral brachiolarian 

 notch obeys similar rules, but we must note that this notch when 

 present occurs between arm-rudiments I. and II., not I. and Y., 

 of the disc. 



(11) Kelation of hydroccde to larval stalk. — In Asterias riihens 

 at metamorphosis, retraction of the preoral lobe to the oral aspect 

 of the disc takes place rapidly through the action of muscular 

 fibres developed beneath the ectoderm and in the eoelomic walls. 

 In Solaster the process, though slower, is eifected by muscular 

 fibres which pass from the wall of the preoral coelona to the oiul 

 aspect of the disc. In both cases the gap (see under 10 above) 

 between the dorsal and ventral horns of the hydrocoele crescent 

 allows the stalk to be dragged into the concavity of the crescent 

 and to be clasped for a time by the hydrocoele ring as the latter is 

 completing itself. In Crihrella the hydrocoele, being practically 

 from the first a small complete ring or disc, does not have the chance 

 of enclosing the larval stalk. HoweVer, a set of retractor nauscle- 

 fibres similar to those in Asterias and Solaster makes its appearance 

 at metamorphosis, passing to the centre of the oral surface super- 

 ficially to the hydrocoele and dividing into branches, some fibres 

 from which pass deeply towards the wall of the gut, while others 

 diverge interradially. At the place where the retractor muscle 

 arises from the wall of the preoral coelom, a slight outpouching 

 can be recognised as early as the 12th day, and later (18th day) 

 a very distinct pocket from the preoral coelom passes towards the 

 centre of the disc in interradius I./V. (see PI. II. fig. 10). Had 

 the hydrocoele in Crihrella been an open crescent instead of a 

 closed ring, the retractor fibres above-described would in all likeli- 

 hood have drawn the stalk and its cavity within the grasp of the 

 hydrocoele crescent, as occurs in Asterias., Asterina, and Solaster. 

 We must look upon the early closed condition of the hydrocogle 

 in Crihrella as less primitive than its open formation in Asterias, 

 In the latter, it seems entirely probalile that the ontogenetic 



