The contractile vacuole is single and
is located at the pointed end, which is directed backwards during
locomotion. One specimen found free swimming among some algae.

Length 104 mu; greatest diameter 36 mu.

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.

[Illustration: 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