380 Miss O. M. Rees on Actinostola callosa. 



off any muscle-folds. The ectoderm of the tentacle is very 

 thick and uniform, with very numerous nematocysts and with a 

 definite nerve-layer lining its inner surface. The nerve-layer 

 is clear and wide around the tip, becoming- narrower near the 

 base. The endoderm is very thin, and here also are numerous 

 nematocysts. The ectoderm of the oral disk shows a few 

 small nematocysts and gland-cells, and has a definite nervq- 

 layer at the base of its cells. The mesoglcea is peculiar, 

 being divided into two layers. The upper consists of a net- 

 work of elongated fibres, the section having been cut parallel 

 to the muscle-strands. The other layer shows a series of 

 much denser fibres with small cavities, this being a cross- 

 section of the muscle. In each case the fibres are arranged 

 parallel to the surface of the oral disk. We thus have two 

 layers of muscles, running practically at right angles to one 

 another and parallel to the surface of the disk. 



This specimen is found to possess nearly all the characters 

 described by McMurrich for Actinostola callosa and by 

 Hertvvig for Dysactis crassicornis. My specimen, however, 

 is much smaller in size than any of those previously described. 

 McMuvrich mentions in Actinostola callosa the beginning of 

 a sixth cycle of mesenteries, and also showing gonads on the 

 mesenteries of the fifth cycle. 



In the Falkland specimen the mesenteries of the fifth cycle 

 are still very small, and there are no traces of any mesenteries 

 arising to form a sixth cycle : this being probably due to the 

 fact that the specimen is a young one. McMurrich, in his 

 generic definition, says : " The tentacles are short and stout, 

 fluted, and with their longitudinal musculature embedded 

 in the mesoglcea." Hertwig also, in his ' Supplement to 

 Actinaria^' says that the tentacles are fluted. 



The Falkland specimen shows no Huting either in pre- 

 served specimen or in transverse section. On comparing 

 the figures of transverse sections of tentacles drawn by 

 Hertwig for Dysactis crassicornis and by McMurrich for 

 Actinostola callosa, it is seen that Hertwig shows a definite 

 series of furrows, and even the mesoglcea is distinctly corru- 

 gated ; whereas in McMurrich's figure there is only a slight 

 folding of the ectoderm. Thus the fluting on the tentacles 

 is a variable character, dependent on the age and state of 

 contraction of the animal. There is also a difference in the 

 musculature of the oral disk of this Falkland specimen as 

 compared with that of Hertwig's specimens. Hertwig found 

 no indications of the presence of two sets of muscle-fibres. 

 This character, however, is not of sufficient specific importance 

 to be of any weight in deciding the identity of the specimen. 



