Spermiogenesis in the vermetid gastropod Dendropoma petraeum (Gastropoda, Prosobranchia).

The structure and maturation of the male gonad of the Mediterranean vermetid gastropod Dendropoma petraeum are described. Histological sections of the gonads were made throughout development and gonad activity was monitored at regular monthly intervals. During the autumn months the gonad is very small and is surrounded by a large quantity of connective tissue; it becomes more voluminous from December to August, with the highest growth peak in springtime. The stages of spermatogenesis were also observed and described.

Engagement of the periesophageal ring during Holothuria polii response to erythrocyte injection.

In Holothuria polii, the periesophageal ring is an important organ supplying spherule cells after stimulation with foreign material. In animals injected with formalinized sheep erythrocytes, in fact, a depletion of spherule cells is observed in the periesophageal ring, whereas in the connective tissue, in the external epithelium and around the antigen-injected site, small, transparent cells can be visualized. It is supposed that the latter are stem cells of spherule cells.