Asymmetry in the Plutei q/" Echinus miliaris. 173 



In both the courses of events indicated by 1 and 2 the following three 

 conditions may possibly arise, according to the different stages at which the 

 right hydrocoele had arrived, when the recovery of the left hydrocoele took 

 place : — 



(a) The recovery of the left hydrocoele takes place before the right hydro- 

 coele attains a size equal to that of the left. The period during which the 

 hydrocoele is deprived of communication with the exterior is very short. 

 Under such a condition the larva in which the left hydrocoele is more 

 advanced than the right will result. This is very frequently met with among 

 double hydrocoele larvae. 



(&) The left hydrocoele recovers at the time when the right one has 

 attained the size about equal to it. The larvae developed under such a condi- 

 tion has two hydrocoeles equal in their states of development. Such a case is 

 less frequently met with than the former. 



(c) The left hydrocele recovers late when the right one is already in a more 

 advanced state than it. The period during which the hydrocoele is deprived 

 of communication with the exterior is here very long. The result is that the 

 larva has the left hydrocoele less developed than the right. Usually the 

 hydrocoele cannot remain unchanged for so long a time after being deprived 

 of its external communication. The case is therefore met with very rarely. 



The above may not be the only ways of reaching the respective results, but 

 probably are the commonest. Many modifications are naturally conceivable. 



Whether the extraordinary development of a hydrocoele from the right 

 anterior coelom is to be regarded as a case of atavism or as other variation is, 

 from the poor state of our knowledge at present, a matter of choice. 

 MacBride ((5), pp. 240, 244 ; (6), pp. 341-45) and v. Ubisch ((16), pp. 444-45) 

 are of the opinion that it is a case of atavism. Grave ((4), p. 43) made the 

 objection to this view that it necessitates the admission of such an improbable 

 fact that the Echinoderm ancestor had a double set of spines and dental 

 apparatus. According to MacBride, the appearance of the right hydrocoele 

 to a certain stage of development is an atavistic feature, but Ikter the 

 formation of the amniotic invagination and those calcareous structures is due 

 to the action of a hormone emanated from the hydrocoele ((5), pp. 247-48 ; 

 (6), pp. 342-43). If this peculiar ability has been acquired by the left 

 hydrocoele during the later period of Echinoid history, as suggested by 

 V. Ubisch, then how could it be transmitted to the right hydrocele now ? 



Gemmill, discussing the causation of the double hydrocoele in starfish 

 larvae ((3), p. 72), introduces the idea of homoeosis, which I prefer to the 

 atavistic view. The difficulty of accepting the idea of homoeosis in such a 



