REPOKT ON THE ACTINIARIA. 9 



The same questions recur in the tentacles, which are merely evaginations of tin- 

 oral disk. Here the endodermal circular muscular fibres are always uniform, whilst the 

 ectodermal longitudinal cords vary. Moreover, there are usually, if not always, open- 

 ings present in the tentacles through which water is ejected when the animal becomes 

 contracted; they occupy the point of the tentacles, and are easily observed in the 

 living animal. In order to find them out in the spirit material I fastened a tentacle, 

 which had been cut off, to a tube and inflated it with air underwater; if an opening 

 were present the air bubbled out through it. 



According to their shape the tentacles are distinguished as "knobbed," "club-shaped," 

 "branched," "conical," &c, terms which do not recpiire further explanation. Their 

 mode of arrangement, of which I shall speak in connection with the septa, is also of 

 importance. On the other hand, their length and shortness is a characteristic which is 

 not capable of exact definition, and cannot be determined with any certainty in the 

 spirit specimens, as it is impossible to judge to what extent the length has been 

 influenced by a greater or lesser degree of contraction. This characteristic cannot, how- 

 ever, be dispensed with for systematic purposes. 



Whether the tentacles in the Actinias may be entirely wanting, without being 

 morphologically replaced in some way or another, seems to me questionable, as no such 

 case is known up to the present time. The tentacles may, however, undergo a peculiar 

 retrograde metamorphosis, progressing so far that only the terminal opening is left in the 

 form of a fissure, which is enclosed by thickened lips, and, lying in the periphery of the 

 oral disk, shows the spot where we might have expected to find the tentacle. I have 

 observed different stages of this retrograde formation in species of Actinias coming from 

 great depths. We see the beginning of it in Polysiphonia tuberosa (PI. II. figs. 7, 9), 

 also Sicyonis crassa (PI. IV. fig. 4), and the advanced stages in Pohjopis striata 

 (PI. II. fig. If), and Polystomidium patens (PI. V. fig. 6). 



The oral opening is only exceptionally round ; it has usually the form of a fissure 

 whose longitudinal diameter lies in the same direction in all Anthozoa. It is therefore of 

 the greatest importance for distinguishing the axes which may be drawn through the 

 body of the Actiniae. If the Actiniae were animals possessing perfect radial symmetry, 

 then the longitudinal axis, determined by its passing through the oral and aboral poles, 

 would be the only constant one, and all radial axes lying perpendicular to the 

 longitudinal would then be perfectly equivalent to one another. By the constant 

 form of the oral opening, the radially symmetrical fundamental form becomes more 

 definite, and is at least transformed into the biradially symmetrical form. In all cases, 

 two of the radial axes strike us as specially distinguishable, the sagittal axis running 

 in the direction of the oral fissure, and the transverse axis perpendicular to it. We can 

 even exceptionally recognise a dorsal and a ventral side at the ends of the sagittal axis, 

 and a right and a left side at the ends of the transverse axis, and hence the 



(ZOOL. CHALL. EXP. PAUT XV. 1882.) 1' -' 



