16 AETHUR DENDY. 



cylindrical tubes penetrating the sponge-wall at right angles 

 to its two surfaces — not extending, however, completely from 

 surface to surface, but terminating at either end just beneath 

 the cortex (figs. 23, 25 — 27). In transverse section (figs. 

 28, 29) the chambers appear approximately circular, or at all 

 events more or less rounded in outline, and not, as in many 

 Sycons, polygonal from mutual pressure. The retention of 

 the primitive cylindrical character is doubtless due to the fact 

 that the chambers are not very closely packed, but separated 

 by a fair amount of intervening mesoderm. At their peri- 

 pheral ends the chambers terminate blindly beneath the dermal 

 cortex, there being, as already stated, no dermal ostia to place 

 them in direct communication with tlie exterior. At their 

 peripheral ends also the chambers exhibit a marked inclination 

 towards branching. I have endeavoured to represent the most 

 striking instance of this which has come under my notice in fig. 

 23. This tendency towards branching of the chambers appears 

 to be not very uncommon amongst the Sycons, and is mentioned 

 by Schulze in the case of Sycandra raphanus (11). I hope 

 to be able to discuss its possible significance at a later date. 



In specimens, or in those parts of specimens which contain 

 pretty far advanced embryos, the walls of the flagellated 

 chambers are frequently seen to exhibit little shallow pits on 

 their inner surface (fig. 23, em. c). These little pits or 

 pockets, instead of being lined by the usual collared cells, are 

 lined by flattened pavement-cells. They are the remains of 

 cavities in the mesoderm from which embryos have escaped 

 by bursting through the wall of the chamber and tearing away 

 part of it with them. The collared cells of the part torn away 

 first become stretched out and flattened, as shown in fig. 38, 

 by the pressure of the growing embryo beneath them ; finally 

 they appear to degenerate altogether, so as to form a structure- 

 less membrane, which is carried away bodily by the escaping 

 embryo. For further particulars as to the mode of escape of 

 the embryos the student is referred to my paper " On the 

 Pseudogastrula Stage in the Development of Calcareous 

 Sponges" already cited. 



