Length 104µ; greatest diameter 36µ.

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 _Opalinidæ_. 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 _Plagiotomidæ_
peristomial furrow having an
adoral zone along the entire
left edge