It is then, if lucky, eaten by some crustacean, or insect, more rarely by a fish.
The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish.
The most aberrant type of larva is that of the genus Prosopistoma, which was originally described as an entomostracous crustacean on account of the presence of a large carapace overlapping the greater part of the body.
No Crustacean exhibits this trilobite form.
Somite corresponding to the eyes (the morphological nature of which is discussed below), the smallest number of head-somites so united in any Crustacean is five.