132 I. IJIMA : HEXACTINELLIDA. I. 



nuclei (fig. 39, tr.n.) are indistinguisliable from the trabecular 

 nuclei, but quite distinct from the choanocyte nuclei with which 

 they may lie side by side. The edge of the membrane around 

 the aj)opyle may be said to be even and free, except for the 

 isolated trabeculie which may sometimes proceed directly from it. 

 At a short distance from the free edge, the marginal membrane 

 passes into the reticular membrane and at the same time into 

 the connecting membrane as well. With the latter it is uninter- 

 ruptedly continuous. Into the former it merges rapidly but with- 

 out any definable demarcation, as will best be judged from fig. 

 39. I might as well have described the connecting and the 

 marginal membrane as one and the same part with which the 

 reticular membrane comes into juncture. 



Of quite usual occurrence is the fact that the chambers are 

 somewhat contracted at the marginal membrane ; so that, when 

 looked at from either the outside or inside of the apopyle at that 

 end, this appears surrounded by a narrow ring of the marginal 

 membrane in a manner that reminds one of the velum in 

 Craspedote Medusae (fig. 22, m.m.). As seen in optical sections 

 passing lengthwise through consecutive chambers, the opposite 

 side-walls of two adjoining chambers run down close together, 

 or perhaps in direct apposition, toward the rims, finally to diverge 

 more or less from each other when they come to the marginal 

 membrane. Shortly before they freely end, the connecting 

 membrane, likewise in optical section, stretches across between 

 them. However, it must not be thought that such a state is 

 invariably found. Sometimes the chamber-wall runs straight out 

 when they come to the marginal membrane, which may then, in 

 certain parts of its circumference, lie in contact and probably in 

 fusion with the marginal membrane of the neighboring chamber. 



