SITUS INVERSUS IN ECHINOIDS 133 



We need not explain the cause of homoeosis in this way only. 

 The chance by which the double hydrocoele is induced to 

 develop seems to be quite unusual, as I will try to show 

 presently. It is not at all a result of adaptation. 



In his famous experiments on Alpheus, .Przibram 

 showed that if a large claw of this Crustacean is amputated 

 a small claw will appear at the spot, whilst the small claw 

 of the other side, which was not operated upon, will become 

 a large claw. This phenomenon he calls ' compensatory 

 hypertypy '. For more detailed information I refer to his 

 later paper (22). A similar but slightly different idea can be 

 applied in the case of double hydrocoeles. The right hydro- 

 coele might have arisen as a result of compensatory hypertypy 

 caused by the arrested state of development in the left hydro- 

 coele. The differences from the case with Alpheus are 

 that (a) the presence of a rudimentary right hydrocoele is not 

 a normal feature, but no doubt the right anterior coelom has 

 a potentiality of producing it, while the small claw of Alpheus 

 is present constantly and quite functional, and (b) the left 

 hydrocoele has not yet been fully developed but arrested in 

 its early stage of development, while the large claw of Alpheus 

 was removed after it had reached the full-grown state. With 

 these differences kept in mind we may use Przibram's 

 term in our case as well. 



According to Eunnstrom (25, p. 305) the further dif- 

 ferentiation of the hydrocoele, left or right as the case may be, 

 depends largely on the formation of an amniotic invagination. 

 There was, however, an exceptional case (Case 24). Besides, 

 from lack of a corresponding amniotic invagination and from 

 obliteration of the dorsal pore, the hydrocoele and its associated 

 structures will degenerate from hunger (Runnstrom, 25, 

 p. 265 ; Mac Bride, 15, pp. 339, 340). 



The presence of two hydrocoeles on one side was noticed by 

 Mac Bride (Case 21), and interpreted as being due to the 

 splitting of the hydrocoele bud. Another curious abnormality 

 was described by Runnstrom (Case 22). There are, accord- 

 ing to this observer, two possibilities as to the cause of such 



