ON MEELIA NORM AN I. 685 



collar-cells, such as occur in the youngest specimen. The 

 reasons for this sharp separation between hypersome and 

 hyposome now seem to me obvious. Tiie calcocytes migrate 

 down and accumulate iti the base of the sponge, leaving but 

 little or no room for the other elements, which can onlv 

 expand upwards. The continued growth of the soft tissues 

 thus separated into upper and lower, or superficial and basal, 

 within the confined space of calcareous pits or tubes which have 

 gradually been forming, leads to an hour-glass constriction 

 and forming of tabulte, Avith still more complete separation of 

 the hyper- and hyposomal elements (see Section 5 below). 



Often the calcocytes niigrate to the periphery of a crypt, 

 leaving tlie central part of the maltha clear and free of cells, 

 and giving to the mass of cells the appearance of beino- 

 arranged as a columnar epithelium against the crypt-wall an(J 

 surrounding a cavity (PI. 34, fig. 4). Partly for this reason I 

 was led to think that Merlia might be a coral of some sort, 

 since both the hard and soft tissues "conspired" to produce 

 this impression. 



(h) 1'okocytes. 



At one time I thought that the cylindrical masses of cilco- 

 cytes in the crypts were gemmule cells, but unless they were 

 capable of dissolving the calcareous skeleton, in which most 

 of them were interred and cut off from the Avorld, it was diffi- 

 cult to understand what could be the object of forming 

 gemmule cells in nearly closed crypts, roofed in by tabulae 

 with central holes or slits sometimes onh' from 1 to 3 ^ in 

 diameter. 



Just below the choanosome in one section are some granular 

 cells with the nucleus and nucleolus larger even than in the 

 calcocytes (PI. 34, fig. 8). Such cells may be egg-cells. The 

 specimen from which the section was made was captured in 

 May. 



(4) Young Stage of Merlia. 

 The youngest example found was in the form of a little red 



