Animal Diversity Web

The cephalaspids are considered the most primitive of the opisthobranch orders. They share certain basic characteristics with the Prosobranchia, but they are placed with the Opisthobranchia because there is a smoother transition from the Cephalaspidea to the other opisthobranch orders.

Haminoea linda, from the South Pacific.

The cephalaspids are characterized by a large head shield, that prevents sand and debris from entering the mantle cavity as the animals plow throught the substrate. In the center of the head shield are two "eyes," pigmented light- sensitive organs.

Species of the family Acteonidae have large shells into which the complete animals can withdraw, an operculum, a streptoneurous (crossed visceral nerves) nervous system, and an external, non-invaginable, male copulatory organ. These are prosobranch characters. But, other characters are opisthobranch-like.

Haminoea linda, from the South Pacific.

The shells of these and similar opisthobranchs are transparent and colorless. The color and pattern seen here is on the mantle and is showing trough the clear, transparent shell. When the animal dies, the shells often wash up on beaches and are hard to distinguish from the bubbles produced by waves splashing on the shore, hence the name "bubble shells."

Smaragdinella calyculata, from the South Pacific.

In the Anaspidea there is a tendency for parapodia to enlarge and, together with the mantle, to enclose the fragile shell (with increasingly reduced and involute spire). There is also a tendency for the mantle cavity to be reduced in size and pushed to the right, and for the visceropallium (visceral mass and mantle) to be dominated by the head-foot.