Streptosyllis varians Webster & Benedict 1887

Head quadrangular, width nearly double the length, the line of separation between the head and the palpi very indistinct, posterior margin slightly concave. There are four eyes, dark red, posterior pair largest, a little behind the middle line, lateral circular, this pair may be divided so as to give one or even three pairs, but one pair is the normal number anterior pair minute, just outside the bases of the lateral antennae. The lateral antennae are club-shaped, their length is about equal to the width of the head. median antenna from two to three times as long as the lateral, they may be club-shaped or cylindrical. The buccal segment is nearly as long as the second segment, it encroaches slightly on the head. The tentacular cirri are like the lateral antennae, but a little shorter, the inferior shorter than the superior.

The ventral cirri are but little shorter than the dorsal, very stout at the base, conical, acute, wrinkled, they arise near the base of the foot and project beyond it, or they may be turned backward.

The setae are compound, with one, or rarely two simple setae in each bundle. The compound setae of the anterior feet have very short terminal articles. Both the stem and the appendix become somewhat longer, going backward, and retain their length on the posterior segments. The appendix is truncate, with two minute terminal teeth.	The simple setae are longer on the segments back of the first third than on the anterior segments, while the curvature is greater on the anterior than on the posterior segments, and on the posterior segments they are nearly straight.
There is a single acicula in each foot, very stout, and with a large terminal button, which barely projects from the foot.

The appendix of the compound chaetae in this species lacks the covering membrane found on the setae of Streptosyllis arenae.

The pharynx occupies six segments, and terminates in front in a circle of triangular papillae. The stomach is narrow, occupies eight segments, narrows slightly behind and passes into the intestine without the intervention of any peculiar glands, but the intestine in the first segment back of the the stomach is colorless. The body is convex above, flattened below, middle third of uniform width, tapering uniformliy but gradually along the anterior and posterior thirds.

The body is colorless, the intestine dark brown to black; pharynx, light brown; stomach, white.

Dimensions of the adult non-sexual forms: length, 6-8mm; width 0.6-0.8 mm.

♂: The capillary (sexual) setae begin on the male on the twenty-first setigerous segment, they are short, not reaching beyond the dorsal cirri, delicate; they fail only on two or three of the posterior segments. The body back of the stomach is much swollen, pure white, but retaining the median. intestinal, brown band.

♀: The sexual setae of the female also begin on the twenty-first setigerous segment, but they are very long and delicate. The eggs are few, large, irregularly polygonal, pure white.