340 ALFRED GIBBS BOURNE. 



I am not aware that any of these forms can be distinguished 

 from one another by observation of their structure, but it 

 would be most interesting to discover whether all or some of 

 them only can become sexual individuals. 



I believe that the number of segments in the sexual indi- 

 vidual is constant for the species, while that in asexual indi- 

 viduals is indefinite; but that w is a number constant for the 

 species, and that the number of segments in z' is constant for 

 the genus, n and z' being used with the signification above 

 described. 



The segments represented by z' are referred to in the 

 generic definitions as the cephalized segments : in most cases 

 all these cephalized segments remain marked in the adult; in 

 all cases the most anterior of them, the peristomial segment, is 

 obvious, while the others are usually marked by the non-deve- 

 lopment of dorsal seta bundles; but in Pristina, at any rate, 

 the bud shows that there are really seven cephalized segments 

 {i.e. z' consists of seven segments), a fact which could not be 

 ascertained by an inspectiou of the well-grown head region. 



The following table represents six zooids or chains of zooids 

 belonging to the same species, an imaginary species in which 

 I have supposed w=10, and z' to consist of three segments ; all 

 the other letters have the signification above described. 

 Instead of actually drawing the segments I have represented 

 them by some numeral. 



