Part 14 (1/2)

Length 104; greatest diameter 36.

I was much surprised to find this form swimming about freely in the water; its mouthless condition showed it to belong to the family of parasites, the _Opalinidae_. As the name indicates, however, this species is an ectoparasite upon the gills, and Stein gave the name _branchiarum_ to a fresh-water form parasitic upon _Gammarus pulex_.

The Woods Hole form is so strikingly similar to the figure of _G.

branchiarum_ that, although the name was given to a fresh-water form, it obviously applies to this marine variety. One important difference is the presence of only one contractile vacuole in the marine form.

[Ill.u.s.tration: Fig. 44.--_Anoplophrya branchiarum_.]

KEY TO FAMILIES OF HETEROTRICHIDA.

Cilia cover the body 1

Cilia reduced to certain 2 localized areas

1. _Polytrichina_.

a. The mouth terminates a long Family _Plagiotomidae_ peristomial furrow having an adoral zone along the entire left edge

b. Peristomial area a broad Family _Bursaridae_ triangular area ending in mouth

c. Peristomial depression short; Family _Stentoridae_ limited to the anterior end; its plane at right angles to the long axis of body; surface of peristome striated and ciliated; no undulating membranes

2. _Oligotrichina_.

a. Peristome without cilia; cilia Family _Halteriidae_ limited to one or more girdles about body

One marine genus *_Strombidium_

b. Thecate forms; the body is Family _Tintinnidae_ attached by a stalk to the cup; within the adoral zone is a ring of cilia.

c. The peristomial depression is Family _Ophryoscolecidae_ deep and funnel-like; cuticle thick, with posterior spine-like processes.

* Presence at Woods Hole indicated by asterisk.

KEY TO THE MARINE GENERA OF PLAGIOTOMIDae.

Diagnostic characters: The peristome is a narrow furrow which begins, as a rule, close to the anterior end and runs backward along the ventral side, to the mouth, which is usually placed between the middle of the body and the posterior end. A well-developed adoral zone stretches along the left side of the peristome, and is usually straight.

1. Body cylindrical; size medium; Genus _Metopus_ peristome long and turns sharply to the left at the extremity

2. No torsion in the peristome; Genus _Blepharisma_ undulating membrane is confined to the posterior part of peristome

3. No peristomial torsion; Genus _Spirostomum_ body highly contractile; no undulating membrane

KEY TO THE MARINE GENERA OF BURSARIDae.

Diagnostic characters: The body is usually short and pocket-like, but may be elongate. The chief characteristic is the peristome, which is not a furrow, but a broad triangular area deeply insunk and ending in a point at the mouth. The adoral zone is usually confined to the left peristome edge, or it may cross over to the right anterior edge.