SPONGES 



99 



certainty. First, in some species of the genus Leucilla we find 

 elongated chambers opening several together into short excurrent 

 canals formed by folding or evagination of the whole wall of the 

 gastral cavity (Fig. 70 ; cf. Fig. 44, A). Secondly, in other cases the 

 excurrent system owes its origin to the further complication of ex- 

 current chamber ducts such as have been described above in the 

 syconoid type. Thus in Leucandra aspera (Fig. 71) a section of 

 the wall of an oscular tube shows the flagellated chambers close to 

 the margin of the osculum opening either directly or by means of 

 an excurrent duct into the gastral cavity. Further down two 

 or more chambers open by a common duct, which may now be 

 termed an excurrent canal. This condition may be due either 



Fio. 70. 



Leucilla connexiva, Pol., part of a transverse section. E, excurrent canals ; for other points 

 gee description of Fig. 68. (After Polejaeff, Challenger Reports.) x 50. 



to the confluence of excurrent ducts primitively distinct, or to the 

 multiplication of the chambers by division. The further removed 

 any spot is from the oscular margin, the more the excurrent 

 system becomes complicated, until a canal system of a typical 

 leuconoid kind is produced. The excurrent canals may branch 

 frequently, and the incurrent system is correspondingly com- 

 plicated. The chambers, though varying greatly in size and 

 shape, are for the most part small and rounded in form, and 

 open directly into the wide excurrent canals. The canal system 

 when fully developed is thus seen to be of the eurypylous third 

 type. Aphodal and diplodal canal systems are not known amongst 

 Calcarea. A leuconoid type, such as is seen in Leucandra aspera, 

 is the highest development of the canal system in this group. 



