STRUCTURE OF SERIATOPORA, POCILLOPORA, ETC. 393 



complete the division of the area of the calicle into twelve 

 chambers. 



The chambers all have a tendency to have their floors pro- 

 longed into conical pits, but it is only in the case of chambers 

 corresponding to the long mesenteries that these are of any great 

 depth. The mesenterial muscles are attached at the bottom of 

 the chamber-pits ; those of the two mesenteries bearing filaments 

 are very long, reaching to the very bottoms of the deep pits to 

 be attached there. 



To the mesenteries which bear the filaments appear to be con- 

 fined the production of the generative elements. I have seen no 

 trace of generative cells except on these mesenteries. The sexes 

 appear to be separate in Seriatopora stocks, and my specimen is 

 from a male stock. Another remarkable fact in the structure 

 of Seriatopora is that the cavities of its polyps are in communi- 

 cation with one another by means of a canal system forming a 

 network. The network traverses the entire area at the surface 

 between the polyps. It forms a close meshwork in the intervening 

 areas, and has in connection with it here and there a few long, 

 nearly straight, larger canals, which run for the distance between 

 three or four polyps to bring the various regions into more direct 

 connection. The meshwork joins the polyp cavities by means of 

 offsets disposed radially to the centres of the polyp areas, two 

 small radial canals usually entering the periphery of each inter- 

 mesenterial chamber, there should be thus twenty-four canal com 

 munications with each polyp cavity ; but in fact there are usually a 

 few less than this through slight irregularity, about eighteen to 

 twenty-two. The canal system is excavated in the mesoderm and 

 lined by large endoderm cells. It lies all in a single plane parallel 

 with the surface of the corallum. The ramifications of the 

 canal system occupy the grooves on the surface of the corallum 

 between the ridges and small prominences with which it is 

 covered. The radial canals joining the polyp cavities pass be- 

 tween the small projecting points surrounding the margins of 

 the calicles. The only other Zoantharian in which such a 

 vascular network has as yet been described as far as I know 

 is Stylophora digitata, in which it has been shown to exist by 

 Koch,' who figures it only as seen in vertical section. It appears 

 not to be present in all compound Madreporarians, for Heider^ 

 has found nothing equivalent to it in Cladocora. It is very 

 similar to the well-known canal system in Alcyonarians, except- 

 ing that these latter usually ramify in several planes. In Seria- 



' G. V. Koch, "Anatomic von Stjiophora digitata," 'Jenaiscbe Zeit- 

 schrift,' Bd. xi, S. 380. 



2 A. V. Heider, « Die Gattung Cladocora," • Sitsb. der k. Acad, der Wiss.,' 

 1881, S. 662. 



