24 AUGUSTA ÄRNBÄCK-CHRISTIE-LINDE, NORTHERN AND ARCTIC INVERTEBRATES. 



being a distinguishing character, the faculty of forming stellate groups seems thus 

 to be an intermediate feature betvveen the two families. 



The different development of the atrial siphon and the formation of a common 

 cloaca in the Botryllids are no doubt distinguishing characters. But the question is, 

 whether they are valuable enough to justify the subdivision into two families. In 

 Metrocarya. in the young zooid, no distinctly developed atrial siphon occurs, only a 

 transversal opening in the mantle. In the colonies examined no zooid has been 

 observed with siphons opening independently on the surface. 



In Botryllus the atrial siphon is well marked-off both in the young zooid and 

 in the full-grown one, though it opens into a common cloaca as a rule. Exceptions, 

 however, have been observed; for instance in young individuals attached to Zostera, 

 in not fully developed stages, the atrial siphons open directly on the surface, before 

 the formation of stellate groups takes place. In large colonies, especially on the 

 margins, I have not infrequently found zooids that were fully developed and mature 

 opening with both siphons independently on the surface. Both siphons were of 

 rounded form and the margin was plain. Consequently there exist intermediate forms 

 betvveen the Polyzoidce and Botryllidce even in this respect, and hence the dif- 

 ferences in question can hardly be regarded as forming an insuperable boundary be- 

 tween the two groups. 



If we consider what has been mentioned above, it will be evident that certain 

 members of the Polyzoidce agree with certain members of the Botryllidce in practic- 

 ally all respects as to internal structure, and with regard to external differences 

 intermediates have been found. On those grounds the division of the two groups 

 into different families cannot be maintained, the family Botryllidce is to be subordi- 

 nated under the Polyzoidce as a subfamily, Boiryllince. 





