The mesenterial Filaments of the Alcyonaria. 1 3 



pronoimced, so that the band assiimes a Y-form, or it may be com- 

 pletely obliterated , as in fig. 8 (Paralcyoniicm). It is difficult to de- 

 termine its exact form in living specimens, but it is certain that the 

 groove is usually very distinct. 



The nuclei of the band are arranged in two lateral groups to corres- 

 pond with the two external lobes. Between these two gronps is a 

 clearer obscurely triangulär mass , the structure of which I have not 

 been able clearly to make out, but which would well repay investigation. 

 In Gorgonia a few pale rounded bodies may be seen in it , which are 

 apparently nuclei. In Paralcyonium very similar nuclei oceur, and in 

 addition a nnmber of bodies which have the appearance of columnar 

 cells. It is possible that these structures may be some kind of a ner- 

 vous apparatus. 



The second part of the filament consist of ordiuary flattened epi- 

 thelial cells like those covering the faces of the septa. As the support- 

 ing lamella reaches the ectodermic band the entoderm cells spread out 

 on each side in a thin layer covering the back of the band. They spread 

 out laterally for some distance and then end abruptly. Usually they 

 form only a single layer , but in Gorgonia (fig. ü) the cells are in some 

 places more than one layer deep. They diflfer in no respect from the 

 ordiuary entoderm cells, and usually have a flattened form. 



The contrast between the entodermic and ectodermic Clements of 

 the filament is very striking and is so clearly shown in the figures that 

 no description is necessary. In Gorgonia especially (fig. 6) the two 

 kinds of cells are extremely unlike ; in other genera the difference is 

 not so great , but is uevertheless apparent at a ghinee. Of the very 

 marked difference between the cells in their behavior towards staining 

 fluids I have already spoken. 



c) Function. 



The embryological origin of the ectodermic filaments , the absence 

 of >^gland-cells« and of foreign bodies, and the fact that these filaments 

 alone appear in the zooids of PennatuUda (which so far as known take 

 no part in the Ingestion of food) form a rather strong body of evidence 

 that they are not concerned in the process of digestion. Their structure 

 at once suggests that they are organs of circulation, and direct Observa- 

 tion, so far as it goes, confirms this Suggestion. 



If a fuUy expanded living polyp of Alcyotmim or Paralcyonium be 

 examined under the microscope , the currents in the gastric fluid can 

 easily be observed. The general direction of these currents is upwards 



