132 IIIROSHI OIISIIIMA 



is necessary, however, to introduce another supposition to 

 understand how the right hydrocoele in our abnormal case 

 acquired that power of stimulating other tissues, which power 

 was not possessed by the right hydrocoele of the ancestor. 

 In short, even if we accept the view that the Echinoderm 

 ancestor possessed a double hydrocoele, it seems to me that 

 the atavistic interpretation has to encounter with such a diffi- 

 culty as stated above. 



The development of a right hychocoele to such an unusual 

 degree may then safely be regarded as a case of homoeotic 

 variation. The examples of this kind of variation given by 

 Bateson (2, pp. 721-35) should bi! classified at least into two 

 different groups. One group contains the cases characterized 

 by the appearance on one side of a wholly new structure, 

 which is quite unknown in the animal's phylogenetic history, 

 whereas a mirror-image of it is normally present on the other 

 side. Gem mill's 'primary' homoeosis (8, p. 71) seems to 

 be this. A tadpole of Pelobates fuscus with a second 

 spiracle on the right side is an example, and if K u n n s t r o m ' s 

 view is accepted the appearance by self-ditferentiation of an 

 anmiotic invagination on the right side of the sea-urchin larva 

 would be another. The second group comprises those cases 

 where, in obviously paired organs, one member, which is 

 normally vestigial, develops in certain circumstances to the 

 same degree as its fellow. A double-tusked narwahl is the 

 best illustration of this kind. Gemmill's term ' secondary ' 

 homoeosis perhaps denotes the same phenomenon. I feel 

 very doubtful whether the case of our double hydrocoele should 

 be placed under this latter category or under the first. The 

 paired origin of the front teeth in the narwahl is quite obvious, 

 while the presence of a pair of well-developed hydrocoeles in 

 the Echinoderm ancestor will not be accepted unanimously by 

 all zoologists. 



I. do not believe that the development of a double hydrocoele 

 has ' resulted in a larval organization better adapted to the 

 conditions under which the existence of the pluteus is led ', 

 as Grave (9, p. 45) states in his discussion on the homoeosis. 



