Fragmentation and Regeneration
Both the situations mentioned above that is whether occurring naturally or accidentally, can in general, be categorized like fragmentation. Of these, the first one, of natural or innate fragmentation followed by regeneration generating complete individuals has already been covered in the previous sections. We shall here explain the second manner in which accidentally generated body fragments regenerate all the missing parts to produce complete individuals. Regenerative capacity is fairly well-developed and extensive in echinoderms, but to a different degree in dissimilar groups and species. Though echinoids or sea urchins are poorly endowed along with regenerative power, star fishes (asteroids), brittle stars (ophiuroids) and sea lilies (crinoids) have wide capacity to regenerate. Most of these, especially many star fishes and brittle stars, not just only regenerate lost arms or part of the central disc, but even an arm can regenerate a whole animal, including the other arms and the central disc. Some starfishes for example Linckia are able to cast off their arm, which will regenerate into a starfish. Some starfishes and some crinoids (e.g. Ophiactis) even normally reproduce asexually. This includes division of central disc so that the animal divides into two. This is known as fissiparity. The two halves will regenerate the missing half. Specific holothurians (sea cucumbers) are unique in showing what is called evisceration. On encountering any immediate danger of some intruder or enemy these can throw out the large masses of "tubules of Cuvier" attached to respiratory trees, or sometimes even almost all of their viscera by their cloaca and regenerate them later. In some species evisceration is a normal seasonal phenomenon. Spontaneous fragmentation of the body followed by regeneration of the body parts to form a new individual is a general form of asexual reproduction in polychaetes. In several syllids the point in the septa where the segment will fragment is predetermined and is dissimilar from other septa. In such species fragmentation and regeneration are highly organized and each fragment can develop into a complete individual. In some cases the original somite stay large and a head and tail regenerate at each end. These break off and the tail portion grows a new head, while the head grows a new tail.