The body is cylindrical to pyriform, in the latter case broadened anteriorly. Cuticle distinctly marked by longitudinal striations which take the form of depressions and give to the body a characteristic melon shape. The endoplasm contains a number of large refringent granules--probably body products. The nucleus is elongate, somewhat curved, and coarsely granular. A micronucleus lies in the concavity. The cilia are long, inserted rather widely apart along the longitudinal markings. 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 algæ. 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 b. Peristomial area a broad Family _Bursaridæ_ triangular area ending in mouth c. Peristomial depression short; Family _Stentoridæ_ 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 _Halteriidæ_ limited to one or more girdles about body One marine genus *_Strombidium_ b. Thecate forms; the body is Family _Tintinnidæ_ attached by a stalk to the cup; within the adoral zone is a ring of cilia. c. The peristomial depression is Family _Ophryoscolecidæ_ deep and funnel-like; cuticle thick, with posterior spine-like processes. * Presence at Woods Hole indicated by asterisk. KEY TO THE MARINE GENERA OF PLAGIOTOMIDÆ. 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.