134 



THE VOYAGE OF RM.S. CHALLENGER. 



and growing over sand grains and other foreign bodies so that tliey become a part of 

 the investing mass. In some species of Psammaplidium the colony looks simpl}- like 

 a mass of sand orains. 



Eeturning now to the point G. at the origin of the Polyclinidse, we find that the 

 second line of descent leads first of all to the main axis of the Distomidce ; and here, just 

 as in the case of the early Polyclinidse, a series of changes mu.st have taken place, 

 resulting in the formation of large colonies in which the Ascidiozooids were more or less 

 comj)letely embedded in a common investing mass formed by the fusion of their tests. 

 The union of the atrial apertures to form common cloacal cavities did not take place 

 apparently so soon or so completely as in the case of the Polyclinidae, and in many of 

 the Distomidas the atrial apertures of the Ascidiozooids are found opening indepeudentlj' 

 on the exterior of the colony. 



The line leading from G. to the base of the Distomidaj gives off a short side 

 branch upon which Diazona is found. This remarkable form,' although not upon 

 the main axis, is probalily the nearest genus now known to the ancestral form at G. 

 which gave rise to most of the Compound Ascidians. Diazona has the Ascidiozooids 

 still partially independent, their posterior ends only being emliedded in the common 

 investing mass. This form diverged from the main line just before the suppression 

 t>f the internal longitudinal bars took place, as they are present in Diazona although 

 absent in CJiondrostachys,' the next genus which left the main axis. 



6r.s. 



B 



Fig. 22. — Diagrams of ColelUi in the Distomidoe. 

 A. sliowa the petluncuhated colony, nat. size. B. shows one of the Ascidiiooids magnified. 



Farther along a side branch is found which has given rise to Oxycoryiiia, DistapUa, 



' See Savigny, Mdmoires, pp. 35 and 175 ; Forbes and Goodsir, Tntns. Roy. Soc. Ediii., vol. xx. p. 307, 1851 ; 

 and Delia Valle, Rend. d. R. Accad. d. Sci. Fu<. Mat. ISIapoU, Auno xxiii. p. 23, 18S4. 

 2 Maodonald, Ann. and May. Nal. Hint., ser. 3, vol. i. p. 401, 185^. 



