Genus CERATIUM (Schrank).

(Stein '78; Perty '52; Clap & Lach. '58; Bergh '82; Pouchet '83;
Gourret & Roeser '88; Bütschli '85; Kent '81; Senn 1900; Schütt '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 shell. The shell 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. Chromatophores 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, 290µ.

Fig. 25.—Ceratium tripos. [ ENLARGE ]

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 chromatophores. Length 285µ; width 23µ.

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

Fig. 26.—Ceratium fusus.
[ ENLARGE ]

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, Bütschli), although Stein maintained that there is a delicate cuticle-like shell. Chromatophores 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 50µ; width 30µ; thickness 15µ.

Fig. 27.—
Amphidinium operculatum.
[ ENLARGE ]

*Key to Infusoria.*
1.	With cilia		Subclass Ciliata. 3
2.	Without cilia (in adult state) tentacles		Subclass [Suctoria]
3.	a.	Without a specialized fringe of large cilia (ad. zone)	Order [Holotrichida]
	b.	With general covering of cilia + adoral zone	Order [Heterotrichida]
	c.	With cilia on ventral side + adoral zone	Order [Hypotrichida]
	d.	With cilia in region of adoral zone, and about mouth only	Order [Peritrichida]

*Key to families of the Holotrichida.*
A.	Mouth closed except during food ingestion; no undulating membrane			1
	Mouth always open; with undulating membrane			2
	1.	Gymnostomina.
		a.	Mouth terminal or subterminal. Food is swallowed and not introduced by currents	Family [Enchelinidæ]
		b.	Mouth terminal or subterminal; body frequently drawn out into long process; mouth may have specialized framework	Family [Trachelinidæ]
		c.	Mouth central or posterior; pharynx with supporting framework	Family [Chlamydodontidæ]
	2.	Trichostomina.
		a.	Mouth anterior or central; pharynx short or absent; peristomial depression faint or absent	Family [Chiliferidæ]
		b.	Mouth central; pharynx long, tubular; cilia in two broad zones	Family Urocentridæ
		c.	Mouth posterior; form asymmetrical; cilia dispersed or limited to oral region	Family Microthoracidæ
		d.	Mouth anterior or central. Peristomial depression clearly marked.	Paramœcidæ (One genus, Paramœcium)
		e.	Mouth at end of long peristome running along ventral side; body dorso-ventrally or laterally compressed; left edge of peristome with great, sail-like undulating membrane	Family [Pleuronemidæ]
		f.	Mouth and pharynx distinct, posterior; cilia uniform. Parasites in ruminants.	Family Isotrichidæ
		g.	Mouth absent; body vermiform, cilia uniform. Usually parasites.	Family [Opalinidæ]

*Key to marine genera of Enchelinidæ*
Diagnostic characters: Form ellipsoid or ovoid; the mouth is invariably terminal and is usually round—more rarely slit-formed; it is closed except when food is taken. An œsophagus when present is a short, invariably non-ciliated tube which is usually surrounded by a more or less clearly defined buccal armature. The anus is usually terminal. Large food particles are swallowed, never introduced by currents.
1.	Body naked		3
2.	Body inclosed in a shell or coat		7
3.	a.	Cilia uniform about the entire body; body symmetrical	4
	b.	Cilia in the mouth region longer than the others; body symmetrical	5
	c.	Bristles, or tentacles, in addition to cilia	6
4.	Mouth terminal; body ellipsoidal to ovoid		Genus Holophrya
5.	a.	Mouth terminal; body elongate, flexible, and elastic	Genus Chænia
	b.	Mouth terminal; "neck" highly elastic; entire body flexible; conical "head"	Genus [Lacrymaria*]
	c.	Mouth terminal; "neck" highly elastic; entire body flexible; "head" square	Genus [Trachelocerca*]
	d.	Mouth terminal; "neck" highly elastic; no separate mouth-bearing portion	Genus Lagynus
6.	a.	Body asymmetrical; bristles in addition to cilia	Genus Stephanopogon
	b.	Body symmetrical; 4 small tentacles from mouth; cilia and cirri in girdles	Genus [Mesodinium*]
7.	Shell composed of small sculptured pieces; cilia long, uniform		Genus [Tiarina*]
* Presence at Woods Hole indicated by asterisk.