Streptosyllis arenae Webster & Benedict 1884

Body ca. 2.8 mm long, for 40 chaetigers; maximal width 300 µm without parapodial lobes, 440 µm with parapodial lobes and 570 µm including chaetae (5^th segment). Head semi-circular, wider than long, anterior margin regularly rounded. Two pairs of eyes, one pair of eyespots, the latter located near the anterior margin (near eye spots). Three antennae present, each irregularly wrinkled, median antenna originating between two pairs of lateral eyes in middle of prostomium, lateral antennae located near anterior margin of prostomium. Median antenna ca. 370 µm long, lateral ones ca. 120 µm.

Palps form two rounded lobes, fused basally, outer margins prolonged into a vestigial cirrus. Palps directed ventrally, not visible dorsally.

Peristomium indistinct, bearing two pairs of tentacular cirri, each about as long as lateral antennae. Dorsal cirri about same length as lateral antennae, some longer, generally shorter near posterior end. Shape and structure of dorsal cirri varies within same animal: they can be smooth, wrinkled, pseudo-articulated with granular rounded inclusions, articulated with each article divided longitudinally or articulated without divided
articles.

Ventral cirri slightly wrinkled, digitiform, wide at base, about length of parapodial lobe in first segments, then becoming longer, about twice length of parapodial lobe in posterior part and longer than dorsal cirri.

Pygidium with a median anal cirrus, lateral anal cirri not observed. Length of median anal cirrus varies from very short to about as long as dorsal cirri among three paratypes.

Single dorsal chaeta present from chaetiger 1, slightly curved, ca. 55 µm anteriorly, 90 µm posteriorly, tip bluntly rounded and covered by a hyaline hood.

Up to ten hemigomph compound chaetae in each parapodium, length ca. 45 µm anteriorly, 85 µm posteriorly. Shaft of compound chaetae ending in up to three rounded teeth (Webster and Benedict report four teeth, not observed in the material examined), sometimes with notched tips. Blades of two kinds, longer ones (ca. 19 µm) in the dorsal chaetae, shorter ones (ca. 12 µm) in ventral chaetae, becoming longer towards the posterior end. In first five chaetigers only chaetae with short blades present; from chaetiger 6 onwards, two long-bladed and 8–9 short-bladed chaetae per bundle present. Longer blades unidentate with rounded tip covered by a halo-shaped hyaline hood, and small tooth near distal part. Shorter blades with a more acute tip and hood prolonged down sides.

Aciculae distinctively enlarged in chaetigers 2–5, with one per parapodium, each distally knobbed, terminating at tip of parapodial lobe.  Length of aciculae ca. 130 µm in anterior chaetigers, 190 µm in posterior chaetigers.

Pharynx unarmed, extending from anterior end to chaetiger 3. Proventriculus
extending through 6–7 segments, with about 56 muscle cell rows.