65 



formation of the ooecial cover and in Porella saccata we meet witli an ooecial 

 cover with many layers, because thin calcareous plates are constantly growing 

 over the ooecium from the three surrounding zooecia. A similar, many-layered 

 ooecial cover seems also to be the rule in the family Reteporidae. 



An ooecial operculum (PI. XXIV, figs. 6, 7, 9, 10, 13, 14, 16 o.o.) has up to now only 

 been found in Bicellaria ciliata, Bugula Sabatieri\ Callopora (Membranipora) Fle- 

 mingi ^ and Microporella Malusi ^ and it consists in these forms only of an evagination 

 from the proximal zooecium's membranous wall, as a rule distally to its oper- 

 culum. Into this evagination extends a muscle, which is able to withdraw it 

 and thus open the ooecium, both for the egg to enter the latter and for the larva 

 to escape. The ooecial operculum seems to be quite lacking in Porella saccata 

 and in the members of the family Reteporidae, and it is very likely to counter- 

 balance this, that the ooecia in this family are furnished with a more or less 

 developed, screen-like continuation, directed inwards, of the rim of the opening. 

 In Thalamoporella we find a well chitinized ooecial operculum, which at its proxi- 

 mal part is connected with the zooecial operculum, and which seems to be pro- 

 vided with a muscle on each side (PI. VI, figs. 7 e, 7 o, 7 n, 7 g, 7 h). 



3) The peristotnial ocecia. These ooecia, which only consist of a single cal- 

 careous layer and have no covering membrane, are formed by the peristome, 

 and may otherwise be very different in shape. In the family Tubucellariidae^ 

 (PI. XVI, figs. 8 b, 4 a, 4 b, 5 a, 5 b, 5 d) they have the form of an irregularly 

 pear-shaped expansion open at the end, while in the genus Lekythopora (PI. XVI, 

 figs. 6 a, 6 b, 7 a) they form a semi-globular expansion on the frontal wall of the 

 long tube-like peristome. I must for the present also refer the helmet-shaped or 

 cup-shaped ooecium in the genera Holoporella and Conescharellina (PI. XXIII, 

 fig. 8 a) to this type. 



4) The endotoichal ocecia. These ooecia which are only found in the genera 

 Cellularia (PL VII, figs. 4 a— 4 f, PI. VIII, 1 a— 1 c, 2 a— 2 c) and Membranicellaria 

 (PI. VII, figs. 2 a— 2 c) are cavities in the thick frontal wall of the zooecium, and it 

 looks as if they are formed by a gradual resorbtion of the calcareous material of 

 this wall. Thus by grinding longitudinally the older and younger parts of a colony 

 we can find these ooecial cavities in all possible sizes, from quite small ones, situ- 

 ated in the middle of the thick wall, up to a size which takes up the largest part 

 of the thickness, and opens outward. According to the investigations of Calvet^ 



' 9, p. 57. ' 9, p. 262. ' 9, p. 169. * My examination of these ooecia has been made on dried 

 material, but Waters has later (116) given a full account of them, based on fresh spirit material. 

 ' 9, p. 264, pi. VI, fig. 11. 



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