7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 33 



glyph. Large nematocysts and gland cells and a distinct nerve layer are present 

 in the ectoderm of the stomodseum. Within each mesenterial space (ento- and 

 exo-coelic) there is an invagination of the polypal wall and in the lower parts 

 of the polyp these meet in the middle and fuse, thus dividing that part of the 

 coelenteron into distinct chambers each of which contains a mesentery. The 

 mesenteries and these infoldings are perforated by the synapticula, and des- 

 mocytes are usually numerous around these perforations. The mesenteries, how- 

 ever, extend but a short distance downwards into the region of the synapti- 

 cula as here they are to a large extent resorbed. Mesenterial filaments occur 

 on the 3 cycles of mesenteries but are usually imperfectly developed on mem- 

 bers of the third cycle; the filament has in section only one lobe which is 

 composed chiefly of ciliated cells, large clear gland cells and cells containing 

 nematocysts of at least 2 kinds. A regular circulation of the fluid in the 

 coelenteron is maintained, up the column wall across the disc and down the 

 sides of the stomodseum into the central part of the coelenteron. Gonads are 

 present on the primary and secondary mesenteries. Larvae probably escape 

 by rupture of the tentacles. They are usually pear-shaped, about 2 mm. long 

 and uniformly ciliated. The broader oral pole is posterior during swimming 

 and is deeply pigmented due to zooxanthell<e in the ectoderm. At the other 

 end the ectoderm is nervous. If the larvse do not fix themselves within the first 

 few days after extrusion they seem to be afterwards incapable of doing so. 

 "Aggregated colonies" [see Bericht for 1902 Coel. p 21] were formed by a 

 number of larvse settling down in close union with one another. The 6 pairs 

 of primary mesenteries are established at the time of fixation of the larva and 

 4 pairs were connected with the stomodseum. Later the 2nd and 5th and the 

 1st and 6th mesenteries on each side form isocnemic pairs and the 3rd and 

 4th constitute the ventral and dorsal directives. After an interval of a month 

 the 6 pairs of secondary mesenteries begin to appear in the dorsal, lateral and 

 ventral exocoels (in this order). During the next 2 months in which the polyps 

 were observed, no 3rd cycle of mesenteries appeared, but in buds the members 

 of the 3rd cycle appear in dorso-ventral succession, 6 pairs in the dorsal 

 exocoels and a pair in some of the ventral exocoels of each system, but this 

 third cycle is seldom, if ever, complete. No polyps have been found with a 

 4th cycle. The 6 primary exotentacles, which appear simultaneously soon after 

 fixation of the larva, are equal in size and remain simple. Internal to these 

 and after an interval of 2 or 3 weeks, 6 primary entotentacles appear and 

 later other entotentacles are formed alongside the latter, then a common pe- 

 duncle is formed which elevates the 2 moieties so as to produce the bifurcate 

 tentacles of the adult. The 2nd cycle of mesenteries having now appeared, a 

 new cycle of exotentacles is formed on the ventral aspect of the primary exo- 

 tentacles so that the outer cycle now consists of 12 exotentacles equal in size 

 and all simple. The 6 secondary entotentacles then appear (at first simple, 

 then bifid) and as the pairs of 3rd cycle mesenteries develop an exocoelic and 

 an entocoelic tentacle arise in connection with each. These entotentacles (simple, 

 then bifid) form a new cycle, whereas the exotentacles become interpolated in 

 the outermost cycle among the previously-formed exotentacles. As the hexameral 

 plan of the 3rd cycle of mesenteries and tentacles is usually incomplete the 

 tentacular formula is 6, 6, x (all entotentacles), 6 + 6 + x (exotentacles) where 

 x may be from 1 to 12. The growth of the septa is not marked by any con- 

 spicuous interruptions such as are characteristic of that of the mesenteries and 

 tentacles. 3 or 4 days after fixation of the larva 6 primary entosepta appear 

 and at the same time a narrow peripheral calcareous ring from which the septa 



Zool. Jahresbericht. 1904. Coelenterata. f 



