CLASS III. TENTACULIFERA. 803 



border of the periphery, leaving two small orifices open only for the passage of 

 the tentaculate appendages. In Atineta mystacina a series of triangular plates or 

 valves, having slit-like interspaces for the exit of the tentacles, protect in a like 

 manner the more ordinarily exposed anterior margin of the body. Some difference 

 of opinion exists respecting the significance and point of origin of the tentacles 

 of the Acinetidae. By some authorities, including Hertwig and Koch, it has 

 been represented that these organs are independent of and perforate the external 

 cuticular membrane, being produced from the central endoplasmic region of the 

 body. Stein, Zenker, and Fraipont, on the other hand, maintain that the tentacles 

 consist essentially of tubular prolongations of the external cuticle only. Still 

 more recently, M. E. Maupas, in his important memoir devoted to the con- 

 sideration of this group,* has arrived at the conclusion that, while in the majority 

 of cases these appendages are direct prolongations of the cuticular region, in certain 

 instances, including more particularly the members of the genus Hemiophrya, 

 the suctorial tentacles originate, as interpreted by Hertwig, from the deeper 

 endoplasmic layer. So far, notwithstanding the number of species carefully 

 examined, the author has been unable to connect the origin of the external or main- 

 wall of the tentacles of either description belonging to this class with any other than 

 the cuticular element, though in all instances their axial substance or central cavity, as 

 the case may be, is in direct communication with the endoplasm or internal body- 

 substance. 



While the extension of the tentacles would appear to be accomplished by the 

 injection into them of the fluid protoplasm of the body, their retraction is visibly 

 brought about, in many species, and most probably in all, through the medium of an 

 exceedingly delicate membranous fibrilla which is developed spirally throughout the 

 length of the tentacles, in some species apparently on the inner, and in others on 

 the external surface. When the tentacle is fully extended, the fibrilla is usually so 

 straightened and attenuate as to be imperceptible, while conversely, on being retracted, 

 it is thrown into more or less closely approximated transverse folds. Podophrya 

 elongata, as recently examined by the author, has been found to illustrate this type of 

 differentiation in a marked degree, while an apparent homologue of this contractile 

 element is' developed in a yet more conspicuous manner in the proboscidiform organ 

 of Ophryodendron. As intimated by Fraipont, the fluid contents of the tentacles, 

 discharged by their contraction or derived by suction from captured prey, on arriving 

 at and being emptied into the soft protoplasmic substance of the body, continue, 

 as the necessary resultant of the forces set in motion, in a straight line with the 

 axis of the tentacle, producing the optical appearance in many instances that 

 these appendages, as interpreted by Hertwig, really originate at some distance 

 within the central protoplasm. In Acineta tuberosa an identical aspect is 

 brought about by the fact that the two lateral bundles of tentacula are, when 

 contracted, invaginated to a considerable distance beneath the surface of the 

 cuticle. 



The phenomena of food-ingestion as manifested by the typical Tentaculifera or 

 Acinetidse are highly remarkable. Their food, which consists mainly of other ciliate 

 Infusoria, is seized by the adherent infundibulate or capitate extremities of the 

 extended tentacles, a perforation in the cuticle of the captured prey at the point of 

 contact is almost immediately effected, and the fluid or semi-fluid contents of the 

 victim's body are transferred by suction in a continuous stream through the tubular 

 tentacle into the body of the Acinetan. While this inges,tive process is recognisable 

 in most instances, and while in many cases the tubular character of the tentacles is 

 so well defined that the two walls of the tube are clearly shown in optic section 

 when viewed with sufficient magnification, in some species, such as Spharophrya 

 magna, the axis of the tentacle, according to Maupas, consists of clear homogeneous 

 sarcode continuous with that of the body, and the incorporation of the protoplasmic 



* " Contribution a 1'Etude des Acinetiens," ' Archives de Zoologie Experimentale,' torn, ix- 

 No. 3, November 1881. 



Z 2 



