48 SUMMARY OF CURRENT RESEARCHES EELATIHG TO 



tlie enteric canal in a definite position relative to the zooecium, and 

 that the cords serve to convey stimuli from one animal to another, as 

 the walls of the zooecium are particularly thin at their points of 

 insertion into the rosette-plates. In Memhranipora, as in Flustrd 

 memhranacea, the funicular plate consists -of a plexus of spindle-shaped 

 cells, of the same size as those of the cords. 



The tentacular sheath is, histologically, a lamella, in which no cell- 

 boundaries can be made out, although distinct cell-nuclei are deposited 

 in it. Although the fibrous cords on the sheath do not appear to 

 contain any nuclei, there can be no doubt that they are muscular. The 

 vaginal sphincter has a more complicated structure in Membranipora than 

 in the forms described by Nitsche or Vigelius. It is half as long 

 as the invaginated part of the ectocyst ; internally to the chitinous tube 

 there is a layer of large cylindrical cells provided with distinct nuclei ; 

 in the lower and inner side of the diaphragm formed by the cylindrical 

 epithelium there is a layer of circular muscular fibres, and the author 

 thinks that there are also a few longitudinal fibres. Nitsche was 

 incorrect in supposing that the tentacular sheath passes directly into 

 the substance of the sphincter. The tentacles and the circular canal 

 consist of three layers of tissue — the outer epithelium, the homogeneous 

 cylinder which corresponds to the muscular tunic of phylactolsematous 

 Polyzoa, and the internal very loose investment of cells. The cylinder 

 is the support of the whole tentacle ; on the side turned away from the 

 mouth it is drawn into two ridge-like processes, which pass at their base 

 into the homogeneous lamella of the circular canal ; from this canal a 

 homogeneous membrane is continued to the enteric canal, of which it 

 forms the outer, firm support. No distinct cells can be made out in the 

 inner loose tissue, but scattered nuclei, surrounded by protoplasm, may 

 be observed. Here Freese agrees with Salensky in thinking that the 

 cavities of the tentacles and their circular canal represent a vascular 

 system, although he has not been able to prove a connection between 

 the circular canal and the body-cavity. 



In a number of points the structure of the enteric canal agrees with 

 that of the Flustreeidse. 



Membranipora, like F. membranac^a, has an organ which appears to 

 be the homologue of what is undoubtedly the nerve-centre in the 

 Phylactolaemata ; it lies on the anal side of the circular canal, and has 

 the form of a triaxial ellipsoid ; the outer membrane does not appear to 

 consist of cells, as described by Yigelius in F. membranaceo-truncata, 

 but is a cuticular secretion of the internal substance. On the whole the 

 author's account agrees very closely with the anatomical descriptions of 

 the chief writers on the structure of the Polyzoa. The paper concludes 

 with an account of the species found in the Baltics 



y. Braohiopoda. 



Modified Ectoderm in Crania and Lingula.* — Miss A. Heath ha^ 

 some observations on a tract of modified ectoderm in Crania anomala and 

 Lingula anatina. This tract is found on the arms of Crania, over the 

 whole of the sides of the tentacles and the fold which lie nest each 

 other, and at the outer base of the fold. When specially modified the 

 portion lies in a groove in the subepithelial tissue, and the epithelial 



* Proc. Biol. Soc. Liverpool, ii. (1888) pp. 95-lOi (3 pis.). 



