Cadlina laevis has a flattened, fragile oval body up to 3.2 cm in length. It is translucent white with opaque white or lemon-yellow pigmentation around the mantle edge. This species has distinctive white or lemon-yellow glands towards the mantle margin. The upper surface bears small conical, opaque white tubercules The underside is covered in a tracery of fine white markings. The front part of the foot (propodium) is groved to form two lips (bilaminate). Its white lamellate rhinophores are short and tapered, often with a yellow tip. The oral tentacles are short, broad and flattened. This species usually has 5 (rarely 6 or 7) tripinnate gills.Cadlina laevis was the first British nudibranch noted to reproduce by direct development (Thompson, 1967). Thompson (1967) observed that the embryonic period lasted for ca. 50 days with a vestigial veliger phase of 7 days occurring within the eggs. Tiny adults emerged from the eggs and remained benthic, having sufficient food reserves to sustain life and apparent growth for over a week before active feeding commenced (Thompson, 1967). Cadlina laevis feeds on encrusting sponges primarily Halisarca dujardiniand Dysidea fragilis. Interestingly, individuals of this species found in the north of the British Isles tend to have a milky white pigmentation and are mainly found in the littoral feeding on Halisarca dujardini. Individuals in the south and western parts of the British Isles have the distinctive lemon-yellow pigmentation and are mainly found feeding in the sublittoral on Dysidea fragilis (Picton & Morrow, 1994; Picton, 2001). It has been suggested that these two distinctive colour morphs may be separate species but the variation may be clinal due to its reproductive strategy (Picton & Morrow, 1994).  

This species spawns in March and its penis is armed with numerous tiny hooked chitinous spines. The tripinnate gills of this species retract simultaneously into a common branchial pit, a process known as cryptobranchiate (Thompson, 1988).