[Illustration:  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, Buetschli), 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 50 mu; width 30 mu; thickness 15 mu.

[Illustration:  Fig. 27.—­Amphidinium operculatum.]

KEY TO INFUSORIA.

1.  With cilia Subclass Ciliata. 3

2.  Without cilia (in adult state) Subclass 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

1. Gymnostomina.

a.  Mouth terminal or subterminal.  Family Enchelinidae
Food is swallowed and not
introduced by currents

b.  Mouth terminal or subterminal; Family Trachelinidae
body frequently drawn out into
long process; mouth may have
specialized framework.

c.  Mouth central or posterior; Family Chlamydodontidae
pharynx with supporting
framework

2. Trichostomina.

a.  Mouth anterior or central; Family Chiliferidae
pharynx short or absent;
peristomial depression faint
or absent

b.  Mouth central; pharynx long, Family Urocentridae
tubular; cilia in two broad
zones