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The discovery of conspicuous populations of the rare species Petrobiona massiliana is reported from shallow water karstic caves of Sardinia. The morphological comparative analysis versus previous data from the entire geographic range highlights that spicular traits seem to be highly conservative in contrast with a notable plasticity of growth form. Lifestyle and adaptive strategies that favoured the successful spread of P. massiliana in karstic caves and its persistence as extremely isolated populations in a fragmented habitat are discussed together with potential environmental constraints. The data indicate that conservation of this Mediterranean palaeoendemic species with a spot-like distribution may be a challenge for the western Mediterranean Marine Protected Areas to confer protection to numerous co-occurring cave-dwelling species.

At the end of spermiogenesis in Chalcides ocellatus tiligugu the acrosomal complex is constituted by a homogeneous anterior cap derived from the acrosomal vesicle and by a paracrystalline posterior cap that arises from nuclear membrane proliferation. During the spermiogenetic process, the spherical nucleus becomes elongated and cylindrical in shape, and assumes a homogeneous appearance because its chromatine condenses. At the onset of spermatid elongation, a set of microtubules appears as a helix surrounding the nucleus. At the end of elongation, the microtubules become parallel to the main axis of the nucleus. From the early stage of spermiogenesis, the two centrioles (proximal and distal) and the flagellum are situated at the posterior pole of the nucleus. The two centrioles are encircled by the dense connecting piece, which is continuous with the nine accessory fibers surrounding the flagellum. At a later stage, the mitochondria form the mitochondrial sheath and four rings of electron-dense material. In the first portion of the flagellum, between mitochondria and accessory fibers, a fibrous helix encircles the entire length of the flagellum. The spermatozoa mature completely eliminating their cytoplasm in the epididymal duct.

The karyology and karyotype analysis of the Afrotropical species Dugesia aethiopica is reported. A double chromosome complement is displayed in a single individual, triploid (3n -27 + 1-2 B-chromosomes) and diploid, (2n = 18 + 0-1 B-chromosome) with a basic number of n = 9. The occurrence of a double chromosome number, involving both somatic and germ cell lines, fits the condition of a diffuse mosaicism or mixoploidy. The coexistence of heteromorphisms between homologous chromosomes, polysomic and monosomic conditions, and the presence of B chromosomes could be considered a condition of aneuploidy. The constant re-occurrence of mixoploidy in D. aethiopica represents an adaptive mechanism whose persistence could be favoured by the asexual reproductive mode. The new finding confirms the existence of two different chromosomal lineages in African planarians with n = 8 and n = 9.

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