1. Hydrobia ulvae, shell height 6mm. If a male is placed on its left side in water, the penis (arrowed) is briefly visible as the animal stretches out to right itself. Extend exposure by holding the spire with forceps, or by inserting it into plasticine.
2. Hydrobia ulvae, shell height 6mm. If a male is placed on its left side in water, the penis (arrowed) is briefly visible as the animal stretches out to right itself. Extend exposure by holding the spire with forceps, or by inserting it into plasticine.

Hydrobia ulvae, shell height 6mm. If a male is placed on its left side in water, the penis (arrowed) is briefly visible as the animal stretches out to right itself. Extend exposure by holding the spire with forceps, or by inserting it into plasticine.