31 



towards the centre of the zooecium. hi Haswellia anstraliensis and Hasw. coronata 

 the frontal wall is furnished with numerous pore-canals, each ending inwardly 

 in a uniporous rosette-plate and as the rosette-plates belonging to the marginal 

 pore-canals have their place in the outer part of the lateral walls, these rosette- 

 plates thus form a connecting link between the common lateral rosette-plates and 

 those belonging to the frontal wall. In species of the genus Myriozoum the whole of 

 the frontal wall is furnished with closely placed, posteriorly directed sac-like pore- 

 canals, each ending in a uniporous rosette-plate (PI. XIX, fig. 16 a). In each canal is 

 a chord of mesenchymatous tissue, which has a club-shaped widening towards the ro- 

 sette-plate. In Sclerodomus denticulatus (PI. XIX, fig. 18 a, 18 b) the frontal wall of the 

 zooecium is furnished with numerous curved, sac-like and widened pore-canals, which 

 contain a similar chord of mesenchymatous tissue. Finally, I may mention that 

 in all the species which have marginal pores and at the same time a median 

 avicularium proximallj' to the aperture, the aviculariuni stands in connection with 

 the first or sometimes also with the second pair of superficial rosette-plates, respec- 

 tively through two or four shorter or longer canals, which issue from the distal 

 part of the avicularian chambers. This is the case for instance with most species 

 of the genus Porella. 



All other so-called pores in the Cheilostomata are, as Per gens has already 

 remarked, not real pores, and when this writer' states »ces petits pores sont, en 

 realite, des cavites intersqueletliques occupees par du tissu epidermique, en con- 

 nection avec les parties squelettiques et avec le parenchyme<s this so far agrees 

 with my observations, as I have always found the bottom of these pores closed 

 by a membrane, which adheres directly to the calcareous skeleton and may with 

 age be calcified to a more or less extent. In some of the species, for instance in 

 ^Lepralia^ Pallasiana, a larger or smaller number of these pores may eventually 

 become closed, and in Smittina porifera they may assume a very different appea- 

 rance according to the different manner in which calcification proceeds. This 

 membranous area in Microporina borealis is divided into a number of small areas 

 by radiating calcareous ridges. The difference between such a pore and a rosette- 

 plate is therefore only, that the latter is furnished with one or more very fine 

 perforations, which are absent in the former. The pores as well as the marginal 

 rosette-plates may as time goes on become surrounded by small chambers, and 

 for instance in Smittina porifera and ^Lepralia^ Pallasiana a meshwork of ridges 

 is formed over the whole surface. Regarding the pores of the ooecia, I need 

 only remark that they are similar to those of the zooecia. 



' 93, p. 308. 



