DINOFLAGELLATA.] PROTOZOA 859 within the common jelly or test (compared by S. Kent to the mesoderm- cells of a sponge-colony) ; (/, similar zooid multiplying by transverse fission ; e, normal zooids with their collars contracted ; /, hyaline mucila ginous common test or zoothecium ; g, individual contracted and dividing into minute flagellate spores (microgonidia) comparable to the spermato zoa of a Sponge. Genera. Salpingwca, James Clark (sedentary, Fig. XXI. 6, 7) ; Laycnceca, S. Kent (free swimming) ; Polyceca, S. Kent (cups united socially to form a branching zocecium as in Dinobryon). ORDER 3. GELATINIGERA, Lankester. The cell-units secrete a copious gelatinous investment and form large colonies. Genera. Phalansterium, Cienk. (Fig. XX. 12) ; Proterospongia, Saville Kent (Fig. XXI. 15). [The Choanoflagellata were practically discovered by the Ameri can naturalist James Clark (68), who also discovered that the ciliated chambers of Sponges are lined by collared cells of the same peculiar structure as the individual Choanoflagellata, and hence was led to regard the Sponges as colonies of Choanoflagellata. Saville Kent (69) has added much to our knowledge of the group, and by his discovery of Proterospongia (see Fig. XXI. 15, and description) has rendered the derivation of the Sponges from the Flagellata a tenable hypothesis.] Further remarks on the Flagellata. Increased attention has been directed of late years to the Flagellata in consequence of the researches of Cienkowski, Biitschli, James Clark, Saville Kent, and Stein. They present a very wide range of structure, from the simple amoeboid forms to the elaborate colonies of Volvox and Proterospongia. By some they are regarded as the parent-group of the whole of the Protozoa ; but, whilst not conceding to them this position, but removing to the Proteomyxa those Flagellata which would justify such a view, we hold it probable that they are the ancestral group of the mouth-bearing Corticata, and that the Ciliata and Dinoflagellata have been derived from them. One general topic of importance in relation to them may be touched en here, and that is the nature of the flagellum and its movements. Speaking roughly, a flagellum may be said to be an isolated filament of vibratile protoplasm, whilst a cilium is one of many associated filaments of the kind. The movement, however, of a flagellum is not the same as that of any cilium ; and the movement of all flagella is not identical. A cilium is simply bent and straightened alternately, its substance probably containing, side by side, a con tractile and an elastic fibril. A flagellum exhibits lashing move ments to and fro, and is thrown into serpentine waves during these movements. But two totally distinct kinds of flagella are to be distinguished, viz., (a) the pulsellum, and (b) the tractellum. An example of the pulsellum is seen in the tail of a spermatozoon which drives the body in front of it, as does the tadpole s tail. Such a "pulsellum" is the cause of the movement of the Bacteria. It is never found in the Flagellata. So little attention has been paid to this fact that affinities are declared by recent writers to exist between Bacteria and Flagellata. The flagellum of the Flagellata is totally distinct from the pulsellum of the Bacteria. It is carried in front of the body and draws the body after it, being used as a man uses his arm and hand when swimming on his side. Hence it may be distinguished as a " tractellum." Its action may be best studied in some of the large Euglenoidea, such as Astasia. Here it is stiff at the base and is carried rigidly in front of the animal, but its terminal third is reflected and exhibits in this reflected condition swinging and undulatory movements tending to propel the reflected part of the flagellum forward, and so exerting a traction in that direction upon the whole animal. It is in this way (by reflexion of its extremity) that the flagellum or tractellum of the Flagellata also acts so as to impel food-particles against the base of the flagellum where the oral aperture is situated. Many of the Flagellata are parasitic (some hsematozoic, see Lewis, 70); the majority live in the midst of putrefying organic matter in sea and fresh waters, but are not known to be active as agents of putrefaction. Dallinger and Drysdale have shown that the spores of Bodo and others will survive an exposure to a higher tempera ture than do any known Schizomycetes (Bacteria), viz., 250 to 300 Fahr., for ten minutes, although the adults are killed at 180. CLASS III. DINOFLAGELLATA, Biitschli. Characters. Corticate Protozoa of a bilaterally asymmetrical form, sometimes flattened from back to ventral surface (Diplopsalis, Glenodinium), sometimes from the front to the hinder region (Ceratium, Peridinium), sometimes from right to left (Dinophysis, Amphidinium, Prorocentrum) the anterior region and ventral surface being determined by the presence of a longitudinal groove and a large flagellum projecting from it. In all except the genus Prorocentrum (Fig. XXII. 6) there is as well as a longitudinal groove a transverse groove (hence Dinifera) in which lies horizon tally a second flagellum (Klebs and Biitschli), hitherto mistaken for a girdle of cilia. The transverse groove lies either at the anterior end of the body (Dinophysis, Fig. XXII. 3, 4 ; Amphidinium) or at the middle. In Gymnodinium it takes a spiral course. In Polykrikos (a compound metameric form) there are eight indepen dent transverse grooves. The Dinoflagellata are either enclosed in a cuticular six 11 (Ceratium, Peridinium, Dinophysis, Diplopsalis, Glenodinium, Prorocentrum, &c. ) or are naked (Gymnodinium and Poljkrikosj. The cuticular membrane (or shell) consists of cellulose or of a similar substance (</. Labyrinthulidea) and not, as has been sup- nosed, of silica, nor of chitin-like substance ; it is either a simple cyst or perforated by pores, and may be built up of separate plates (Fig. XXII. 10). The cortical protoplasm contains trichorysts in Polykrikos. The medullary protoplasm contains often chlorophyll and also diatomin and starch or other amyloid substance. In these cases (Ceratium, some species of Peridinium, Glenodinium, Prorocentrum, Dinophysis acuta) nutrition appears to be holophytic. But in others (Gymnodinium and Polykrikos) these substances are absent and food-particles are found in the medullary protoplasm which have been taken in from the exterior through a mouth ; in these nutrition is holozoic. In others which are devoid of chlorophyll and diatomin, &c., there is found a vesicle and an orifice connected with the exterior near the base of the flagellum (cf. Flagellata) by which water and dissolved or minutely granular food-matter is introduced into the medullary protoplasm (Protoperidiniutn pcllu- cidum, Peridinium direrycns, Di2>lo2)salis lenticula, Dinopltysis laei is). It is important to note that these divergent methods of nutrition are exhibited by different species of one and the same genus, and possibly by individuals of one species in successive phases of growth (?). No contractile vacuole has been observed in Dinoflagellata. The nucleus is usually single and very large, and has a peculiar labyrinthine arrangement of chromatin substance. Transverse binary fission is the only reproductive process as yet ascertained. It occurs either in the free condition (Fig. XXII. 2) or in peculiar horned cysts (Fig. XXII. 8). Conjugation has been observed in some cases (by Stein in Gymnodinium). Mostly marine, some freshwater. Many are phosphorescent. The Dinoflagellata are divisible into two orders, according to the presence or absence of the transverse groove. ORDER 1. ADINIDA, Bergh. Characters. Body compressed laterally ; both longitudinal and transverse flagellum placed at the anterior pole; a transverse groove is wanting ; a cuticular shell is present. Genera. Prorocentrum, Ehr. (Fig. XXII. 6, 7); ExumeUa, Cici}k.(=Dinopyvis, Stein; Cryptomonas, Ehr.). ORDER 2. DINIFERA, Bergh. Characters. A transverse groove is present and usually a longi tudinal groove. The animals are either naked or loricate. Fam. 1. DINOPHYIDA, Bergh. Body compressed ; the transverse groove at the anterior pole ; the longitudinal groove present ; longitudinal flagellum directed backwards ; loricate. Genera. Dinophysis, Ehr. (Fig. XXII. 3, 4) ; Amphidinium, Cl. & L. ; Amphisolenia, Stein ; Histioncis, Stein ; Citharistcs, Stein ; Ornithoccrcus, Stein. Fam. 2. PERIDINIDA, Bergh. Body either globular or flattened ; transverse groove nearly equatorial ; longitudinal groove narrow or broad ; loricate. Genera. Protoperidinium, Bergh; Peridinium (Ehr.), Stein (Fig. XXII. 1, 2); Protoceratium, Bergh; Ceratium, Sch rank (Fig. XXII. 15); Diplopsalis, Bergh; Glenodinium, Ehr. ; Hcterocapsa, Stein ; Gonyaulax, Diesing ; Goniodoma, Stein ; Blepliarocysta, Ehr. ; Podolampas, Stein ; Ampliidoma, Stein ; Oxytoxum, Stein ; Ptychodiscus, Stein ; Pyrophacus, Stein ; Ccratocorys, Stein. Fam. 3. GYMNODIXIDA, Bergh. As Peridinida but no lorica (cuticular shell). Genera. Gymnodinium (Fig. XXII. 5), Stein ; Hcmidiniuin, Bergh. Fam. 4. POLYDIXIDA, Biitschli. As Gymnodinida, but with several independent transverse grooves. Genus. Polykrikos, Biitschli. Further Remarks on the Dinoflagellata. This small group is at the moment of the printing of the present article receiving a large amount of attention from Bergh (81), Klebs (83), and Biitschli (82), and has recently been greatly extended by the discoveries of Stein (80), the last work of the great illustrator of the Ciliate Protozoa before his death. The constitution of the cell-wall or cuticle from cellulose, as well as the presence of chlorophyll and diatomin, and the holophytic nutrition of many forms recently demonstrated by Bergh, hashed to the suggestion that the Dinoflagellata are to be regarded as plants, and allied to the Diatomaceae and Desmidiacese. Physiological grounds of this kind have, however, as has been pointed out above, little importance in determining the affinities of Protozoa. Biitschli (82) in a recent very important article has
shown in confirmation of Klebs that the Dinoflagellata do not