218 THE ROYAL SOCIETY OF CANADA 



No matter what basis we use for classification in this family we 

 are confronted with a considerable amount of parallelism. Neigh- 

 bouring sub-families or genera show similar series of genera or species 

 respectively. This makes classification doubly hard, as it permits 

 of alternative very diverse and, at first glance, equally natural schemes. 

 In the past the characters presented by the pharynx have been 

 considered as of major importance in Ascidian classification. This 

 is owing to the diversity of structure exhibited by this organ as well 

 as to the ease with which this structure may be observed. We believe, 

 nevertheless, that in the classification of this family the characters 

 presented by the pharynx are of minor importance to those presented 

 by the gonads. The fact that the pharyngeal characters used for 

 distinguishing the genera are to a considerable extent such as alter 

 from one type to another during the development of the individual 

 makes them on the whole more variable than the gonadic characters. 

 The latter, also, seem to be more fundamental, that is, related to the 

 structure of the germ-plasm. The reason for this may be that the 

 distinctness of such systematic groups as we are here dealing with 

 depends upon the inability of the eggs and sperms of any two groups 

 to unite, that is, crossing of species or genera whose eggs and sperms 

 are mixed promiscuously in the sea water may depend upon certain 

 peculiarities in the structure of these eggs and sperms. If differences 

 in the structure of the gonads exist, these may well be correlated with 

 differences in the structure of the eggs and sperms that are produced 

 in these gonads. 



Seeing that so little attention has in the past been given to the 

 structure of the gonads in connection with classification, it is not 

 surprising that most of the species in this family have not been de- 

 scribed fully enough as regards the gonads for one to be altogether 

 certain, or even in many cases to have any idea of, their position in the 

 present scheme of classification, and comparatively few species have 

 been available to us for examination. Many species, and even several 

 genera, have consequently not been brought into the scheme. Further 

 knowledge concerning these is much to be desired, and will make it 

 possible to fill the gaps and correct the mistakes in this classification. 

 In a number of cases, where there has not been sufficient information, 

 a genus or species has been assigned a position on the basis of its 

 structure so far as known. 



Synopsis of Genera 



Ai. Left ovary in or across intestinal loop, with usually several dis- 

 tinct testicular systems grouped along it. Vasa deferentia not 



