ABSTRACT
In terms of structural diversity, number, biomass, sexual plasticity and modes of reproduction, the invertebrates surpass vertebrates. The endless experimentation undertaken by them with form and function since the early Cambrian has generated about 1.6 million aquatic invertebrate species (of 2.62 million animal species, about one million are terrestrial invertebrates), which are systematically classi ed into eight major (Table 1.1) and 25 minor phyla (Table 1.2). Strikingly, acoelomates like the poriferans, cnidarians, platyhelminthes and eucoelomates, like the annelids and echinodermates are also capable of asexual reproduction. In them, totipotent or pluripotent adult stem cells have been shown to perform agametic cloning and asexual reproduction (Skold et al., 2009). However, pseudocoelomates, Nematoda, Gastrotricha, Rotifera and a few others, and hemocoelomates, Mollusca and
Arthropoda are not known to reproduce asexually. In them, regeneration is limited to one or more tissues (see Table 1.3) but not to a whole animal. Apparently, the presence of Pluripotent Stem Cells (PSCs) is linked to asexual reproduction but in their absence, regeneration is limited to a few tissues but not the whole animal.
