236 M. Sars on tlie Development of Star-fishes. 



velopment is completed within an interval of from six to seven 

 weeks. The perfectly developed young still however abide a long 

 time, at least in one of the examined species [Aster acanthion Mul- 

 leri), within the uterine cavity of the parent, and are so carried 

 about by the latter. In the other species, the Echinaster sangui- 

 nolentus, I have met with young of a radiary form, and with the 

 organs of attachment not yet obliterated within the uterus ; but 

 whether they remain there longer than those of the former spe- 

 cies, or how long, I have not yet been able to ascertain. 



4th. The question presents itself, are the Star-fishes subjected 

 in the progress of development to a metamorphosis or not ? The 

 answer to this will depend on the more or less extended sense in 

 which we please to adopt the term. If we understand by it, with 

 some naturalists, that abrupt transition from one condition of 

 structure to another, in which, as in the passage of the insect from 

 the larva to the pupa state, and this again to the imago, there is 

 a complete change of external form, then indeed the Aster iada 

 cannot be said to undergo any such transformation ; but if, on 

 the other hand, we take the word in the usually received sense, 

 thus expressed by Lamarck*, " Je nomme metamorphose cette 

 particularite singuliere de Pinsecte de ne pas naitre soit sous la 

 forme, soit avec toutes les sortes des parties qu'il doit avoir dans 

 son dernier etat," then we must concede it as fully applicable 

 to the creatures under consideration. For their form, we have 

 already learnt, is in those two stages of development that I 

 have considered to be the earliest, bilateral instead of radiated, 

 and the young enter the world without possessing most of the 

 more important parts belonging to their organization, such as 

 mouth, arms and tentacles, these being produced at a subsequent, 

 later period. Furthermore, parts are developed, for instance, the 

 above-mentioned organs of attachment, which are destined only 

 for immature age, and therefore disappear entirely as the animal 

 approaches to maturity. In this last respect their metamorphosis 

 has been termed retrograde, and offers an example of what Rathke 

 calls " metamorphosis retrograda per dissolution em f." The rea- 

 son for the disappearance of the organs of attachment depends 

 upon their becoming utterly useless, on account of the develop- 

 ment of tentacles with which the young Star-fish begins a new 

 method of locomotion, and enters into other relations with the 

 external world. 



* Hist. Nat. des Animaux sans Vertebres, p. 277, torn. iii. 



f Rathke must have misunderstood me when quoting my observations 

 from Wiegmann's ' Archiv/ 1837, in his 'Travelling Notes from Scandi- 

 navia' : he alleges " that the Star-fishes possess when very young a delicate 

 stem, which proceeds from the middle of their back, and by which they at* 

 tach themselves to other bodies." 



