BRYOZOA. 299 



and that the lenticular vesicles provisionally retain the more 

 appropriate designation "vesicular tissue." 



In this restricted sense the mesopores are tubular cells occu- 

 pying the interspaces between the true zooecia. Their number 

 and size varies greatly in different species, being quite variable 

 even in species of the same genus. In some they are very small 

 and occur only at the angles of junction between the zooecia, 

 while in other forms the latter are completely separated from 

 each other by one or more series. When closely examined they 

 are found to be, perhaps in all cases, more or less decidedly 

 angular and of irregular shape, though it is not infrequently 

 found that the angles are blunted and the cavity rounded by 

 an internal deposit of sclerenchyma. This is the case in all 

 those forms in which the cell walls are thick. Numerous ex- 

 amples might be cited, but Stictoporella and Batostomella will 

 suffice. In longitudinal sections the tubes of the mesopores are 

 generally distinguished from the true zooecia by their compara- 

 tively more numerous diaphragms, which are always horizontal 

 and "complete.'' From a morphological point of view the most 

 important character of the mesopores is found in the fact that 

 they do not occur in the axial region of the zoarium, but first 

 make their appearance in the cortical portion after the original 

 zooecia have commenced to bend outward (or upward as the 

 case may be) in their course to the surface. 



The structure known as "vesicular tissue" is composed of 

 lenticular cells, which fill the interspaces between the zooecia in 

 the FISTULLIPORID.E and CYSTODICTYONIDJS. Unlike the mesopores 

 they originate already in the axial region, being, how r ever, more 

 numerous as well as smaller in the superficial portion of the 

 zoarium. They are never arranged so as to form tabulated 

 tubes like the mesopores, but occur in horizontal layers, the 

 constituent cells of which are quite independent of those of the 

 preceding layer. Near the surface of an adult zoarium they are 

 largely filled with sclerenchyma, which more or less completely 

 obliterates their individuality. This deposit, however, is 

 traversed by minute vertical tubuli, that seem to serve the pur- 

 pose of maintaining communication between the open cells and 

 the surface to which their prominent terminations impart a 

 minutely granular aspect. Vesicular tissue is also developed in 

 the older regions of species of Semicoscinium between the "ca- 



