KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 63. NIO 9. 21 



Remarks. 



It is not intended that a complete diagnosis of the genera Metrocarpa and 

 Botryllus shonld be given in this note. That should be done in conjunction with a 

 re-examination of the other genera belonging to the family Botryllidce. But I call 

 attention to the differences which the present investigation has made evident and 

 established, which make a separation of the two groups necessary and justify the 

 formation of a new genus. 



When we compare Metrocarpa and Botryllus with each other, we observe im- 

 portant differences even in respect to external appearance. In Metrocarpa the grea- 

 test extension of the body is in length, in Botryllus in breadth. In Metrocarpa the 

 branchial siphon is well developed, the atrial one is replaced by a very large and 

 wide opening in the mantle, no true siphon being formed. In Botryllus both the 

 atrial and branchial siphons are well marked-off, making the resemblance to Mon- 

 ascidians more apparent. 



As to the internal organ systems the general structure of the branchial sac is 

 the same in both genera, but differing details have been observed for instance as to 

 shape, number and arrangement of the stigmata. The alimentary canal is of dif- 

 fering structure. As has been pointed out by Michaelsen (1. c.) differences are 

 especially noticeable as regards the folds of the stomach. 



The reproductive apparatus exhibits more essential distinguishing characters. 

 Thus apart from certain common group-characters, for instance the resemblance as 

 to the shape and structure of the testes and the occurrence of one-egg ovaries, the 

 two genera do not at all coincide in respect to this organ system. In Metrocarpa 

 the female organs are situated behind the testes, in the posterior end of the body. In 

 Botryllus the organs are situated almost on a level with each other, the male organ 

 to the ventral side, the ovary to the dorsal side. In Metrocarpa only one fully deve- 

 loped ovary has been observed on each side at the same time. In Botryllus there 

 are generally many ovaries on each side at the same time. In Metrocarpa there is a 

 brood-pouch that develops to receive the embr3'os and is situated on the outside of the 

 body-wall. In Botryllus the embryos undergo their development in the peribranchial 

 cavity, where a special apparatus develops in the form of cup-shaped folds to attach 

 them to the peribranchial wall. 



If we consider the relation of the Botryllidce to other Tunicata, it will be noticed 

 that the structure of the reproductive apparatus offers several interesting points of 

 agreement, not only with closely allied groups such as for instance the Polyzoidce, as 

 will be shown in the following, but also with the Synoicidce, as has been mentioned 

 above, and even with such remote allies as the Pyrosomidoa and the Salpidce. A 

 detailed comparison does not fall within the limits of this note; I may, however, point 

 out the resemblance shown by the occurrence of one-egg ovaries in the two last- 

 mentioned groups, the peculiar structure of which, so far as is known, is not of ten 

 met with. According to Neuman 1 only one ovary occurs in the Pyrosomidce; it is 



1 Neuman, G., Die Pyrosomen in Bronn's Tierreich. 



