Haddon and Shackleton — A Revision of the British Actinice. 619 



however, distinct. Nematocysts are found in the ectoderm of the oesophagus of 

 Palythoa coesia (?), E. couchii, and E. arenaceus. 



The mesogloea of the oesophagus is generally thin, but in many species of 

 Epizoanthus and of Parazoanthus it is much thickened in the groove. Cell enclosures 

 are generally absent in this region, but we have seen a few cell-islets in the 

 mesogloea of the groove of P. anguicoma (one specimen), and they are also present 

 in that of P. dichroicus. 



Mesenteries. — The mesenteries which are such a valuable aid to classification of 

 the Actiniee generally are arranged in this group in a very uniform manner, which 

 we have already described. 



Imperfect mesenteries. — The imperfect mesenteries vary in the extent to which 

 they project into the coelenteron. In E. incrustatus, and in Parazoanthus douglasi 

 they are very small, whereas in P. dixoni they are well developed. When canals 

 are present in the perfect mesenteries they are also to be found in the imperfect. 



Perfect mesenteries. — There is no distinction between any of the perfect 

 mesenteries. 



Ectoderm. — The presence of ectoderm in certain mesenteries of the Anthozoa 

 appears to be now fairly well established. E. B. Wilson (" Mesenterial Filaments 

 of the Alcyonaria": Mitth. Zool. Stat. Neapel, v. 1884) came to the conclusion 

 that the " dorsal " pair of filaments in the Alcyonaria were ectodermal in origin, 

 but that the other six filaments were solely endodermal ; these two kinds of 

 filaments can be readily distinguished histologically. Some years previously von 

 Heider [Cerianthus memhranaceiis, Haime: Sitz. kais. Acad. Wien, Ixxix. 1879) 

 believed, solely from a histological study of the adult Cerianthus, that the filaments 

 of that form were ectodermal. The Brothers Hertwig (Die Actinien, 1879) 

 pointed out that embryological deductions based on adult histology were not very 

 reliable, and also brought forward, as an objection to von Heider's view, the 

 existence of filaments on imperfect mesenteries in the Actinise generally. H. V. 

 Wilson (Journ. Morph. ii., 1888) has shown for the coral Manicina, and J. P. 

 M'Murrich {ibid. iv. 1891) for the Actinian, Rhodactis, that the mesenterial 

 filaments are derived from the ectoderm of the oesophagus. 



From the histological characters and absolute continuity of what we have 

 termed the "reflected ectoderm" of the mesenteries with the ectoderm of the 

 oesophagus on the one hand, and with the mesenterial filament (craspedum) on the 

 other, we have no doubt as to the morphological identity of these tissues. 



Reflected Ectoderm and Filaments. — The mesenteric ectoderm consists of two 

 portions, an upper, which we speak of as the "reflected ectoderm," and a lower 

 portion, which runs down the edge of the mesentery, and is known as the mesen- 

 terial filament, or "craspedum" of Gosse. These two are perfectly continuous 

 with each other and with the ectoderm of the oesophagus. 



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