Fresh and salt water.

Peridinium digitale Pouchet. Fig. 23.

Synonyms: _Protoperidinium digitale_ Pouchet; _Protoperidinium_ Bergh p. p.; _P. divergens_ Peck.

The sh.e.l.l is covered with pits of large size. The posterior part is hemispherical and surmounted by a single horn or spine. The transverse furrow is very oblique, and its two extremities are united by a sigmoid longitudinal furrow. The anterior half bears two spines or horns of different size, and variable. The nucleus is spherical or ellipsoidal and placed in the posterior half of the sh.e.l.l.

Length 68; diameter 54. Common.

Although the description of Pouchet's _P. digitale_ differs in some respects from a careful description of the Woods Hole form, I think the species are the same. The chief difference is in the single horn of the posterior half; in Pouchet's form this is furrowed by a narrow groove which runs to the S-shaped longitudinal furrow. In the Woods Hole form I was unable to make out such a furrow. The flagella, also, were not seen. This same form was pictured by Peck '95 as _P.

divergens_.

[Ill.u.s.tration: Fig. 23.--_Peridinium digitale_.]

Peridinium divergens Ehr. Fig. 24.

Synonym: _Ceratium divergens_ Kent.

The sh.e.l.l is spheroidal, widest centrally, attenuate and pointed posteriorly; the anterior portion is armed with two short, pointed horns, each of them having a toothed process at the basal portion of the inner margin. They are frequently colorless and beautifully transparent, the body being free from large opaque granules; again they are colored brown or yellow. The nucleus is large and elongate and finely granular. 75 long and 68 in diameter. Common.

[Ill.u.s.tration: Fig. 24.--Ventral and dorsal aspects of _Peridinium divergens_.]

Genus CERATIUM (Schrank).

(Stein '78; Perty '52; Clap & Lach. '58; Bergh '82; Pouchet '83; Gourret & Roeser '88; Butschli '85; Kent '81; Senn 1900; Schutt '98.)

The general shape is a flattened sphere with three long processes or horns. The cross-furrow is either spiral or circular; the longitudinal furrow is usually wide and occupies the greater part of the anterior half of the sh.e.l.l. The sh.e.l.l is thick, reticulate or striped, and sometimes provided with short spines; often distinctly porous. The anterior half is composed of 3 equatorial and 3 apical plates, the latter being continued into the horn-like process. The posterior half is composed of 3 equatorial and one apical plate continued into the posterior horn. The right posterior plate is continued into a similar horn which may remain rudimentary or be continued into a considerable process. Similarly the left posterior horn is usually developed, but remains small. There may be from 2 to 3, 4, and 5 horns. Chromatoph.o.r.es usually present, green to yellow brown.

Fresh and salt water.

Ceratium tripos Ehr. Fig. 25.

The body is somewhat triangular and bears three horns, two of which are shorter than the other one and slightly curved upward.

Length, including the horns, 290.

[Ill.u.s.tration: Fig. 25.--_Ceratium tripos_.]

Ceratium fusus Ehr. Fig. 26.

Synonym: _Peridinium fusus_ Ehr.

The animal is very elongate, due to the presence of two long horns at the extremities of the body. Color, yellow with chromatoph.o.r.es.

Length 285; width 23.

Both of these species are common in the tow and in the algae at the edge of the wharf. Both of them are mentioned by Peck in '93 and '95.

[Ill.u.s.tration: Fig. 26.--_Ceratium fusus_.]

Genus AMPHIDINIUM Clap. & Lach.

The body is ovoid to globular and usually much flattened dorso-ventrally. The anterior portion is very much reduced and is somewhat head-like or cap-like. The longitudinal furrow extends through the entire posterior body length and is apparently capable of widening and narrowing. It is probably naked (see here Klebs, Pouchet, Butschli), although Stein maintained that there is a delicate cuticle-like sh.e.l.l. Chromatoph.o.r.es of brown or green colors present and usually grouped radially about a central amylum granule.

The nucleus is posterior.

Fresh and salt water.

Amphidinium operculatum Clap. & Lach. Fig. 27.

The body is oval and flattened. The transverse furrow is at the extremity (posterior) of the body and the small portion, which is thus apparently cut off, is the cap-like or operculum-like structure which gives the name to the species. Klebs maintains that the two furrows are not connected, but in this he is certainly mistaken, provided we have the same species under consideration. Very common about Woods Hole.

Length from 40 to 50; width 30; thickness 15.

[Ill.u.s.tration: Fig. 27.--_Amphidinium operculatum_.]

KEY TO INFUSORIA.

1. With cilia Subcla.s.s _Ciliata_. 3

2. Without cilia (in adult state) Subcla.s.s _Suctoria_ tentacles

3. a. Without a specialized fringe of Order _Holotrichida_ large cilia (ad. zone)

b. With general covering of cilia Order _Heterotrichida_ + adoral zone

c. With cilia on ventral side Order _Hypotrichida_ + adoral zone

d. With cilia in region of adoral Order _Peritrichida_ zone, and about mouth only

KEY TO FAMILIES OF THE HOLOTRICHIDA.

A. Mouth closed except during food 1 ingestion; no undulating membrane

Mouth always open; with undulating 2 membrane