Diagnosis: Congoglanis sagitta differs from C. inga in having a shorter maxillary barbel, not reaching to vertical through anterior margin of orbit vs. extending well past anterior margin of orbit; a shallower caudal-peduncle depth, 6% of standard length vs. 7–8%; and more slender caudal peduncle, its depth 3.1–4.0 times in its length vs. 2.1–2.9; a shorter pelvic fin, 19–21% of standard length vs. 22–25%; a more anteriorly placed anal fin, its origin well in advance of tip of adpressed pelvic fin vs. origin approximately at tip of adpressed pelvic fin; shorter anal-fin rays, posterior rays of adpressed anal fin extend to, rarely slightly beyond, vertical through posterior limit of adipose fin vs. posterior rays extend well past vertical through posterior limit of adipose fin; and more vertebrae, 39–41 vs. 35–37 (Ref. 88958). It differs from C. alula in having a shorter pectoral fin, 23–27% of standard length vs. 28–29%; a shorter pelvic fin, 19–21% of standard length vs. 24–28%; a longer caudal peduncle, 21–24% of standard length vs. 18–22%; and more slender caudal peduncle, its depth 3.1–4.0 times in its length vs. 2.6–3.3 times; a relatively wide interorbital width, 2 times eye diameter vs. 1.5 times; a more anteriorly placed anal fin, its origin well in advance of tip of adpressed pelvic fin vs. origin approximately at tip of adpressed pelvic fin; and more vertebrae, 39–41 vs. 36–39 (Ref. 88958).
Description: A slender species, with body very elongate, nearly cylindrical from head to anal-fin origin, with abdominal region slightly flattened, progressively tapering further posteriorly; greatest body depth and width located at dorsal-fin origin; body smooth skinned and firm (Ref. 88958). Dorsal and ventral profiles of body from head to vertical through anal fin nearly horizontal, but converge from vertical through anal-fin origin to posterior end of caudal peduncle; lateral line complete, running along midlateral surface of body; short, more-or-less uniformly spaced dorsal and ventral branches extend off main portion of lateral line; body without dorsolateral or ventrolateral vertebral processes extending to skin surface from region of adipose-fin base posteriorly to caudal-fin base; caudal peduncle elongate, slender and compressed, but nearly as wide as high at terminus of anal-fin base; its length approximately equal to head length; least depth of caudal peduncle located approximately at posterior of peduncle (Ref. 88958). Head profile acutely triangular from dorsal view with lateral margin slightly convex; snout tip rounded; head pointed and depressed from lateral view; snout moderately long, relatively slender and tapering anteriorly; eye slightly horizontally elongated and situated entirely within posterior one-half of head; without free orbital margin; diameter of eye slightly more than one-half of interorbital distance; distance from posterior naris to anterior margin of eye approximately equal to distance between anterior naris and tip of snout; posterior naris closer to anterior naris than to eye; distance between nares of each side slightly less than one-half distance of each to contralateral naris; dorsal and lateral surfaces of head and body with scattered, tiny , white flattened tubercles, possibly free neuromast organs (Ref. 88958). Mouth subterminal, small, ovoid when open, with fleshy tuberculate upper lip and smooth firm lower lip, flanked to each side by tuberculate section; upper jaw with teeth in broad crescentic patch continuous across midline; lower jaw with teeth in oval patch on each side of symphysis, with narrow median gap; teeth in both jaws long, slender and slightly curved; numbers of teeth in upper and lower jaws about equal; barbels tuberculate and distally tapered; maxillary barbel falling short of vertical through anterior margin of the eye; inner mandibular barbel shortest, with base situated slightly lateral of midline and approximately in line with lateral margin of smooth portion of lower lip; outer mandibular barbel arises from angle of mouth and extends posteriorly approximately to vertical through tip of maxillary barbel; branchiostegal membrane continuous across ventral midline, with central portion of posterior margin straight or slightly concave; gill slit extends posterodorsally to horizontal through eye; supraoccipital process elongate and slightly wider near posterior tip; process separated by small gap from small triangular nuchal shield (Ref. 88958). Dorsal fin falcate, first branched ray longest; tip of adpressed dorsal fin extends past vertical through base of pelvic fin; first dorsal-fin ray segmented, not forming spine; adipose fin origin located at vertical through anal-fin origin; fin small, with length of base approximately one-half distance between posterior terminus of dorsal-fin base and adipose-fin origin; adipose fin adnate, with posterior limit of fin at vertical through tip of adpressed anal fin; pectoral fin large and falcate, with first ray curved, pectinate, broad and with fleshy pad covering ventral surface; distal margin of fin slightly concave, with first ray longest; innermost pectoral-fin rays short, posterodorsally oriented and adpressed to body wall; tip of adpressed pectoral fin falling far short of pelvic-fin origin and falling short of vertical through posterior terminus of dorsal-fin base; pelvic fin moderately large but distinctly smaller than pectoral fin; distal margin slightly concave, with first ray longest; first pelvic-fin ray curved and pectinate, with ventral surface covered by fleshy pad; pelvic-fin origin located distinctly posterior of vertical through posterior end of dorsal-fin base; tip of adpressed pelvic fin extends to, or nearly to, anal-fin origin; anal fin small, with distal margin straight; tip of adpressed anal fin extends to vertical through posterior limit of adipose fin; anal-fin base slightly greater than one-half length of adipose-fin base; caudal fin deeply forked, asymmetrical, with ventral lobe longer and broader than dorsal lobe; fin lobes large and acutely pointed; middle rays of caudal fin slightly less than one-half length of longest rays of dorsal lobe; dorsal-fin rays i,6,i or i,7; pectoral-fin rays i,11-12 or i,11,i; pelvic-fin rays i,5; anal-fin rays iii-iv,5-6,i or iii-iv,6-7; principal caudal-fin rays i,7,8,i; vertebrae 39-41; ribs 12-14 (Ref. 88958).
Colouration: Body colouration generally dark, with variably distinct lighter regions on overall dark brown body; body darker dorsally with brown colouration extending ventral of lateral line on abdomen and caudal peduncle; boundary between dark brown pigmentation of dorsal and dorsolateral regions and lighter colouration of ventrolateral and ventral regions relatively straight and horizontal from pectoral-fin base to caudal peduncle (Ref. 88958). Six pale patches distributed in irregular horizontal pattern along dorsolateral part of body; abteriormost patch ovoid, horizontally elongated, extending just dorsal of lateral line and extending posteriorly to point somewhat anterior to slightly posterior of vertical through dorsal-fin origin; second patch rounded or somewhat square, centered along vertical through posterior terminus of dorsal-fin base; third patch horizontally elongated and slender; patch extending above lateral line from vertical through pelvic-fin origin to vertical through midlength of adpressed pelvic fin; ventral margin of fourth patch variably somewhat more dorsally positioned than dorsal margin of third patch; contralateral fourth patches in contact along dorsal midline and forming saddle immediately anterior of adipose-fin base; fifth patch horizontally elongated and located entirely above lateral line; patch extends from vertical through middle of adipose fin to vertical through posterior limit of that fin; sixth patch horizontally elongated and of variable length on posterior half of caudal peduncle, terminating posteriorly at anterior limit of dark spot at caudal-fin base; dark spot at caudal-fin base semicircular; straight anteriorly and broadly convex posteriorly; spot extending nearly to dorsal and ventral margins of caudal peduncle; ventrolateral portion of abdomen and ventral part of caudal peduncle with scattered dark chromatophores, but chromatophores absent on ventral portion of abdomen as far posterior as pelvic fin; pale stripe extends along entire extent of lateral line (Ref. 88958). Dorsal and lateral portions of head dark brown other than variably shaped pale spot immediately posterior of eye; spot smaller tha, or approximately equal in size to, eye; ventral part of head pale with diffuse region between brown and pale portions ventrolaterally; maxillary barbel with dusky base dorsally, otherwise unpigmented; mandibular barbels unpigmented (Ref. 88958). Dorsal surfaces of pectoral and pelvic fin brown except for unpigmented transverse patch extending across central portion of inner rays and pale distal margin; ventral surfaces of pectoral and pelvic fins without chromatophores except for small patch along basal portion of innermost rays; dark pigmentation on dorsal surface of distal portions of rays appears as dusky region extending along sub-marginal portion of ventral surface of both fins (Ref. 88958). Dorsal fin with dense concentrations of dark chromatophores along rays; interradial membranes sometimes with few scattered chromatophores; adipose fin darkly pigmented basally and dusky more distally; sometimes with narrow dark distal margin; anal fin with dense concentrations of dark chromatophores along rays; interradial membranes of anal fin with dark chromatophores; chromatophores more concentrated on anterior portion of fin; caudal fin with scattered dark chromatophores on all rays, but most concentrated on first and second branched rays of each lobe, giving appearance of dark stripe along each lobe; interradial membranes of caudal fin with few scattered chromatophores (Ref. 88958).