186 ARTHUR DENDT. 



In this species we find a series of very distinct subdermal 

 cavities supported by a special skeleton, for further informa- 

 tion with regard to which the reader is referred to the sections 

 of the present paper dealing with the skeleton and classifica- 

 tion of the Heterocoela. 



We find in some species of this genus also the same tendency 

 to form colonies, by complete fusion of Leuconoid individuals, 

 as we met with in Leucandra. An admirable example of 

 this is afforded by Leucilla (Teichonella) prolifera, of 

 which I gave an illustration in a previous memoir (9), showing 

 the oscula arranged side by side in rows. 



Summary. 



Thus we find that the canal system varies considerably in what 

 appear to be closely related genera of Heterocoela Calcarea; and 

 that, if my view of the relationships of the genera be correct, the 

 Leuconoid type has been independently evolved from the Syco- 

 noid type, along the lines indicated above, no less than three 

 times. The Leuconoid type cannot, however, be produced until 

 the corticate Sy conoid type has been arrived at; and when this 

 condition has been reached the conversion of the originally 

 long and radially disposed chambers into short, rounded, and 

 irregularly arranged ones seems such a simple matter that it 

 may well have taken place again and again. The variation in 

 shape and size of the chambers, even in the same species, may, 

 as 1 have already shown, be very great. The branching of the 

 radial chambers in Syconoid forms is of such common occur- 

 rence in the most diverse genera as to excite no surprise wher- 

 ever we meet with it; and the shortening of the chambers and 

 corresponding elongation of the exhalant canals, due simply 

 to shifting of the limits of the lining of collared cells, has been 

 repeatedly observed in various genera. 



We may sum up our observations on the canal system of the 

 Heterocoela by indicating again the various stages which appear 

 to have been passed through in the gradual evolution of the 

 most complex Leuconoid from the most simple Syconoid type. 



Stage A (Sycetta stage). — The flagellated chambers are 



