This order comprises those Infusoria which, like Peridinium, find their means of locomotion in long, flexible, lash-like filaments, termed "flagella;" cilia occasionally being present as well. In some, as in Peranema and in the Monads (fig. 34, B), there is only a single one of these appendages; in others, as in Anisonema, there are two flagella; whilst in Heteromastix (fig. 34, G) and Pleuronema we have forms apparently transitional between the Ciliata and the Flagellata, since both cilia and flagella are present in these genera. In all their other essential characters the flagellate Infusoria do not differ from the more typical members of the class, with which they agree in possessing a cuticle, a firm cortical layer, and a soft granular central parenchyma, whilst they possess a nucleus and one or more contractile vesicles. They have, however, the peculiarity that, in many cases at any rate, the base of the flagellum is surrounded by a cup-like or cylindrical membranous collar, which can be retracted at will. Many also (as Englena, Astasia, Heteromastix) possess the singular brightly coloured mass of pigment which is known as the eye-spot," and which may possibly be a species of sense-organ (fig. 34). In one singular form (Phalansterium inies-tinale), the organism consists of numerous zooids, each with a single flagellum and projecting membranous collar, enveloped basally in slimy sarcode, so as to form a cylindrical colony.

Fig. 33.   A, Stentor Miilleri; B, Vaginicola crystallina; C, Group of Vorticellae; D, Detached bud of Vorticella, showing the posterior circlet of cilia.

Fig. 33. - A, Stentor Miilleri; B, Vaginicola crystallina; C, Group of Vorticellae; D, Detached bud of Vorticella, showing the posterior circlet of cilia.

Another remarkable animalcule now usually referred to this group is Noctiluca, which occurs abundantly in most oceans, and to which much of the phosphoresence of the sea is due.* It is of large size and spherical in form, with an indentation or "hilum" at one side, where the mouth is situated, and beside which is fixed a single long flagellum. The body consists of a central vacuolated mass of protoplasm, surrounded by a superficial layer, and in turn invested by a thin cuticle. The superficial layer is connected with the central protoplasmic parenchyma by numerous radiating, branched, and anastomosing filaments of sarcode. The luminosity appears to reside in nucleated cellular bodies in the outer layer of the central protoplasm - that is to say, in the peripheral layer of sarcode immediately below the cuticle.

Fig. 34.   Suctorial and Flagellate Infusoria. A, Podophrya; B, Cercomonas truncata; C, Monas neglecta; D, Euglena sanguinea; E, Ccdosiga pulcherrima; F, Astasia trichophora ; G, Heteromastix proteiformis. f Flagellum; m Collar at the base of the flagellum ; c Contractile vesicle; n Nucleus; e Eye spot. (After Pritchard, Ehrenberg, and James Clark.)

Fig. 34. - Suctorial and Flagellate Infusoria. A, Podophrya; B, Cercomonas truncata; C, Monas neglecta; D, Euglena sanguinea; E, Ccdosiga pulcherrima; F, Astasia trichophora ; G, Heteromastix proteiformis. f Flagellum; m Collar at the base of the flagellum ; c Contractile vesicle; n Nucleus; e Eye-spot. (After Pritchard, Ehrenberg, and James-Clark.)

* The diffused luminosity of the sea is mainly due to the Noctiluca miliaris; but its partial luminosity is due to various phosphorescent animals, amongst which are the Physalia utriculus (the Portuguese man-of-war), Medusa, Tunicata, Annelides, etc. The cause of phosphorescence is variously stated, it being supposed very generally to be the result of a process of slow combustion analogous to that which takes place in phosphorus when exposed to the atmosphere. Upon the whole, however, it appears that the phenomenon is a vital process, consisting essentially in the conversion of nervous force (or vital energy) into light; just as the same force can be converted by certain fishes into electricity. This transformation appears generally to require a special apparatus for its production.