Development of the slug

Sexual reproduction
Adults are simultaneous hermaphrodites: each animal, when sexually mature, produces both eggs and sperm at the same time.
Their hermaphroditic gonad is an ovotestis.
The animals do not self-fertilize, however; instead they cross-copulate.
Fertilization is internal. After eggs are fertilized within the animal, the animal lays the fertilized eggs in long strings.

 

Cleavage and gastrulation
Cleavage is holoblastic and spiral. This means that the eggs cleave completely (holoblastic); and each cleavage plane is at an oblique angle to the animal-vegetal axis of the egg. The result is that tiers of cells are produced, each tier lying in the furrows between cells of the tier below it.

At the end of cleavage, the embryo forms a stereoblastula, that is, a blastula without a clear central cavity.

Gastrulation is by epiboly: the ectoderm spreads to envelope the mesoderm and endoderm.

Larval stage
The embryo goes through a trochophore-like stage during development, and then hatches as a veliger larva. The veliger larva has a shell, and it swims using its ciliated velum. The ciliated velum also brings food to the mouth. The larva feeds on phytoplankton in the sea-water column.

The larva has two eyespots on either side of the mouth. Food brought to the mouth by the ciliated velum, is then moved down the digestive tract to the stomach. Here, food is sorted, and then moved on to the digestive gland. It is in the digestive gland that the food is digested and nutrients are absorbed by epithelial cells of the digestive gland. What is left is moved on to the intestine, where mostly water is resorbed.