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.

Genus LACRYMARIA Ehr. '30.