Body (excluding appendages)
Up to 6mm long. Smooth body with no tubercles, occasionally wrinkled. Black [image1] or brown , often with a pale greyish or yellowish quincunx (5 dots as on dice)  , but four lateral spots may be more numerous or merge into lateral lines  , or pattern may be obscure/absent  . When black or brown pigment not dense, dark green digestive gland  or orange ovotestis  may show dorsally. Anus a short distance behind mid point of body (often difficult to see, except when defecating).
Anterior can extend to almost horizontal, or contract to vertical . Dorsally black or brown, intensity varying with extension, and varying amounts of white pigment . Rhinophores and eye patches whitish. Rhinophores digitiform on adults , spatulate on juveniles  . Ridges may run forwards from the rhinophores to front of head (Eliot). On some specimens (at least in Fleet lagoon) basal half of tentacle may extend forwards as a vane ; form is probably that described (thick crests extended posteriorly into two fairly short tentacles), and illustrated at top right of plate 3 (Archive), in Quatrefages's original species description. Sometimes a slight ridge behind rhinophore , this feature prominent (exaggerated?) in Eliot's illustration . Juveniles start to develop rhinophores when about 1.5mm long  & . Sometimes rhinophores contract  , making confusion with L. capitata possible.
Periphery of foot dorsally translucent white with opaque white marks . Sole  translucent pale showing orange ovotestis spheroids, dark green digestive gland and dorsal peripheral white marks. No propodial tentacles or extensions, but anterior of foot slightly swollen laterally and concave centrally. Metapodium pale dorsally.
- Thin digitiform tentacles  on head when 2mm long or larger, with or without anterior ridge or vane . But juveniles lack tentacles or just have ridges above eyes; they are often misidentified as L. capitata or L. depressa.
- Small pale metapodium  & .
- Anus a short distance behind mid point of body.
- Full salinity, also brackish Fleet lagoon, Dorset and variable salinity pools at MHWS on exposed coasts.
- No tentacles or rhinophoral crests, but some have ribs/sharp edges to recessed eye-area, above, in front of and below eye) [1Ld] .
- Pale metapodium (tail) absent or negligible.
- Dorsal anus close to posterior.
- In brackish or fully marine salinity (though slow to adapt to change in salinity).
On its foodplants, Cladophora spp.  in shaded positions and in rock pools. Sheltered lagoons to extremely exposed coasts. All tide levels, including pools of varying salinity at MHWS on exposed coasts. Strands of Cladophora lack cell walls; divided by cross walls into large compartments. Terminal compartments individually emptied of protoplasm by piercing and suction of L. senestra . Hermaphrodite. Vaginal opening closed by bursa copulatrix; often visible as swelling on right side of body . Impregnation of partner by piercing of hypodermic sharp style on end of penis (Gascoigne, 1976). Spawn; February – September . Egg mass contains up to 40 ova; unusually small number, but the largest eggs (diameter 0.4mm) on record, for any opisthobranch . No veliger larval stage; miniature adults hatch from eggs. Only European opisthobranch, apart from Runcina coronata, to have no trace of a shell at any stage of its embryonic development (Thompson, 1976).