DEVELOPMENT OF SOME STARFISHES. 559 



retraction of the preoral lobe and its incorporation with the 

 oral surface of the starfish repeat the phylogenetic changes in 

 virtue of which, after fixation, the preoral lobe disappeared and 

 the mouth, with the oesophagus, migrated to the centre of a disc- 

 like ar-ea which gradually developed radial symmetry. Asterina 

 shows a corresponding but less perfect repetition in which the 

 larval mouth and oesophagus are temporary non-functional struc- 

 tures. Solaster has no larval mouth or oesophagus at all, but the 

 retractor muscle-fibres of the preoral lobe are present and the 

 hydrocoele during retraction forms an open crescent, Crihrella 

 is still further modified : larval mouth and oesophagus do not 

 occur a,nd the hydrocoele is never an open crescent. However, 

 the retractor muscle-fibres and the position of sucker and last 

 remnants of the preoral lobe in interradius I./V. remain as 

 primitive characters still decipherable on the developmental 

 palimpsest of the most crucial pei'iod, namely the fixation period, 

 in its ancestral history. 



(12) Changes of shape in starfish lay'vce. — The larvae of echino- 

 derms in general, and starfishes in particular, undergo remarkable 

 changes of form sometimes quickly, sometimes slowly. These 

 changes are no doubt due in chief part to unequal growth, but there 

 is evidence that contraction of muscular fibres developed ad hoc 

 also plays a part. Thus, as seen above, the abrupt alteration of 

 form at metamorphosis, inckiding the retraction of the preoral 

 lobe, is due to muscular action. I believe that a similar factor 

 operates in many other changes of form if we could only follow 

 out the process. For example, the pulling down of the wall of 

 the gut towards the middle of the oral surface to form the adult 

 mouth appears to be due to an extension of fibres from the pre- 

 oral lobe retractor set (see under 11 above). Fibres from this set 

 pass out interradially, and probably in the end these form the 

 dilating fibres of the mouth which lie within the buccal mem- 

 brane and are attached to ridges on the mouth-angle plates. 

 Phylogenetically, the retractor fibres of the pi^eoral lobe may be 

 referable to a particular segment of mouth-dilator fibres which 

 became hypertrophied, in order at the proper time to be able to 

 drag the preoral lobe after the larval mouth when the latter 

 migrated to the middle of the starfish disc. 



The primary division of the archenteron into anterior, middle, 

 and posterior regions takes place somewhat suddenly and without 

 there being any corresponding external division of the lai'va. into 

 anterior, middle, and posterior segments. At this time muscular 

 tissue has not yet become differentiated, but one may note, in the 

 basal portions of the cells lining certain regions of the archen- 

 tei'on, a staining reaction similar to that given by muscular fibiils. 

 This appears very markedly in the middle or enteric region 

 where naiTowing of the archenteron is taking place, and I think 

 it possible that the staining indicates specially contractile pro- 

 toplasm present for^ the purpose of pioducing the requireol 



