ITS SPECIES AND OVICELLS. 127 



In T. cereoides the end of this curved tube becomes gradually narrower, 

 and the opening is small. The shape of the opening of the ovicell seems 

 to be a specific character, for in T. zanzihmiensis, sp. uov., the end of the 

 tube is straight. 



Many preparations of Mediterranean and other specimens have been made 

 in the hope of finding an explanation of the meaning of these curved tubes, 

 but vi^ithout success, until it was at last revealed in a specimen from Chuaka, 

 Zanzibar. There is some doubt whether this form should be called 

 T. cereoides or should be separated specifically, as the ovicell is smaller than 

 those seen in the Mediterranean specimens ; but at present we can hardly 

 appreciate the value of the ovicellular characters, as so little work has been 

 done in their investigation. 



A number of serial sections have revealed most interesting and unexpected 

 facts, but much remains to be made out, and sections are wanted of other 

 species. The polypide (figs. 20, 25) is much modified, being thin and small, 

 the tentacular sheath together with the diaphragm is extended, and passes as 

 a tube through the oral aperture, past the operculum, to the end of the 

 ovicell (figs. 18, 19) ; and near the diaphragm the lining membrane extends 

 nearly across the opening of the ovicell, ending with a solid cover having a 

 thick cuticula, under which there are some muscles ; and this expanded end, 

 which at present I would call a plug, fits into a small notch at the opercular 

 opening of the peristomial ovicell (fig. 18, see also figs. 20, 21, 25). 



In order to understand the ovicell we should, before going further, examine 

 the position of the tentacular sheath and diaphragm in the ordinary zooecia. 

 This diaphragm has been mentioned by Nitsche, Vigelius, Freese, Jullien (as 

 irisoide), Pergens, Harraer (as vestibule), and by Calvet, who has given a 

 resume of the views of these authors. There seem, however, to be various 

 points which have not been fully appreciated, and some of these may be found 

 to be of use in determining the species. Longitudinal sections with the 

 polypide withdrawn show the diaphragm looking somewhat like a contracted 

 sac, which, in 2\i,hiicella7ia and in many species of other genera, is situated at 

 the side of the tentacular sheath as shown in fig. 15 ; in other cases it lies 

 centrally in the tentacular sheath, as in Myriozoum sahgracile, d'Orb., fig. 17, 

 and here the mechanism is more easily followed, as part of the polypide has to 

 pass through this diaphragm before the tentacles are spread out. The 

 diaphragm or the tentacular sheath is attached on one side to the operculum, 

 and on the other to the zooecial wall (figs. 15, 16, 17), and this does not appear 

 to have been generally understood — or, at any rate, explained. When the 

 polypide is exserted a part of the tentacular sheath is carried with it, as shown 

 diagrammatically in many text-book figures. It will be necessary to deal with 

 these points in other species in subsequent papers. 



In the ovicells of Tuhucellaria the diaphragm is carried far beyond the 

 opened operculum (op), and is attached by a continuation of the tentacular 



