Eyes are open invaginations without crystalline lens.
Well-developed glandular invaginations occur in different positions on the foot in Pectinibranchia.
In some embryos there is but a single pair of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature.
The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs.
Two pairs of invaginations of B the skin, which originally are called the prostomial and metastomial disks, grow round the intestine, finally fuse together, and form the skin and mus- cular body-wall of the future Nemertine, which afterwards becomes ciliated, frees itself from the pilidium investment and develops into the adult worm without further metamorphosis.