Читать книгу Parasitology - Alan Gunn - Страница 44

Many workers consider there to be two groups of animals: the Parazoa and the Eumetazoa. The position of the Placozoa remains enigmatic with some authors including them amongst the Parazoa, some placing them in the Eumetazoa, and others isolating them into their own independent grouping. For many years, Trichoplax adhaerens was the only known placozoan, but at the time of writing, there were three species. Placozoans are small (~1 mm), flat aquatic organisms with a rather amoeboid shape. They glide across benthic surfaces using cilia and absorb algae and detritus across certain cells lining their ventral surface. There are currently no records of them being parasitic or acting as hosts for parasites. However, they harbour rickettsia and bacteria endosymbionts, and it would be surprising if other microbes and viruses did not parasitize them. Interestingly, T. adhaerens is currently the only metazoan animal known to express the protein ‘apicortin’ (Orosz 2018). Apicortins probably help stabilize microtubules and are characteristic of apicomplexan parasites such as malaria and certain free‐living algae to which they are distantly related. It is uncertain whether the placozoans acquired the apicortin genes from consuming algae or sharing genes through long distant evolutionary events.