EEPORT ON THE ACTINIARTA. 7 



Actiniaria ; this is generally most inappropriately expressed by the term " retractile 

 tentacles." It would be decidedly more rational to make the anatomical reason, and not 

 the physiological appearance, of systematic value. We shall therefore talk of Actiniaria 

 without sphincter, and of Actiniaria with weak and with strong sphincter, and further 

 distinofuish in the latter case whether the muscle is endodermal or mesodermal. 



The systematic value of the circular muscle does not end here, as it furnishes a 

 character not to be undervalued, for determining the species. The extraordinary variations 

 of the circular muscle are shown by a glance at Plates VI. and VII. ; in the endodermal 

 forms the shape and mode of branching of the muscular folds vary, in the mesodermal 

 the shape and grouping of the bundles formed by the fibres, and also their position 

 in the more superficial or deeper layers of the wall. I lay stress upon this point, as 

 the circular muscle can be examined in the preserved animals even when their state of 

 preservation is not very favourable, and because, moreover, a small piece of the wall, 

 which can be cut away without essential damage to the whole animal, is sufficient for 

 such an investigation. 



Muscles, especially longitudinal muscles, are rarely present on the ectodermal side of 

 the wall, whilst, on the other hand, it is not unusual to find " marginal spherules " and 

 difierent forms of papillae. The marginal spherules (" bourses marginales," Hollard, Ann. 

 d. Sci. Nat., Zool., ser .iii., t. xv. p. 257) follow immediately outside the tentacles, and 

 are evaginations of the mural membrane, just as the tentacles are evaginations of the 

 oral disk. All the layers of the body participate in the evagination, though the ectoderm 

 alone undergoes modification of its structure, being extraordinarily rich in nematocysts. 



The papillse, to which such importance was attached in earlier investigations of the 

 Actinise, are formations of very subordinate value ; they are caused by mere local growth 

 of the supporting plate, and are not distinguished by a single special property of the 

 covering epithelium (PL VIII. fig. 4). Hence the observer often found himself on the 

 horns of a dilemma when he had to decide w^hether papillse were present or not. A 

 smooth surface may become papillose in consequence of contraction, and, on the other hand, 

 small papillse may disappear when, as often happens, the Actinia becomes distended 

 like a drum. It would, therefore, be better in future only to make the papillose or 

 smooth nature of the membrane of value in distinguishing species, or at most of genera, 

 and to disregard it in the formation of larger divisions. 



The comportment of the epidermis appears to me much more important. The majority 

 of the Actiniae have a smooth surface, on which particles of mucus become secreted 

 when the animal is irritated ; histological investigation then shows an active ciliated 

 epithelium composed of extremely long, thin cylindrical cells. Besides this, two varying 

 modifications of the integument have already been specially observed. In the one case, 

 in Cerianthics, the epithelium is covered externally by a tough membrane, consisting 

 of mucus, nematocysts, and scattered foreign bodies, which can be stripped off, but which 



