(4) The CERCARIÆ[266] are very different; typically they consist of the anterior body and the oar-like tail at the posterior end (fig. 134). The former, even to the genitalia, has the organization of the adult digenetic Trematodes, and thus allows the easy recognition of at least the characters of that large group to which the species in question belongs. On the other hand, however, there are also organs that are lacking in the adult form, such as, in many, the boring spine in the oral sucker, or the eyes situated on the cerebral ganglion; moreover, also, cutaneous glands (fig. 134), the secretion of which forms the cyst membrane. The oar-like tail may be long or short (stumpy-tailed cercaria) or entirely absent; its free end may be partly split (furcate cercaria), or split to its base (_bucephalus_); in various forms also the anterior end of the tail is hollow, and has enclosed within it the anterior body, which is otherwise free. The size also of the cercaria belonging to the different species is very diverse; in addition to forms swimming in the water that have the appearance of minute milky-white bodies, there are forms which measure as much as 6 mm. in length. [266] The cercaria is the characteristic larval stage of the Trematodes, and corresponds to a cysticercus or cysticercoid, though there is the important difference that the cercaria has an enteric cavity. According to some observers the enteron is represented by the frontal sucker of some Cestodes, and by the rostellum of the majority of others. The sporocyst and redia are regarded as intercalated stages, _viz._, as cercariæ exhibiting _pædogenesis_, _i.e._, development of young by a parthenogenetic process from individuals (_i.e._, cercariæ) not yet adult. The encysted cercariæ (fig. 135) are globular or oval, and are surrounded by a homogeneous membrane, which may be striated or contain granules. The tail is always cast off when encystment occurs, and organs peculiar to the cercaria stage (boring papilla, eyes) almost entirely disappear. On the other hand, the genitalia appear or become more or less highly developed, in extreme cases to such an extent that they become functional, and after autocopulation the creatures produce ova within the cysts.