"Thereby Hangs a Tail"

Abstract

Cercariae continue to serve as a dependable basis for taxonomy of the digenetic trematodes when variation in the extent to which certain types may develop in the molluscan host is understood, divergence and convergence of caudal structure is recognized, and greater reliance is placed on the body and excretory system. The tail is considered to be homologous with the posterior region of other generations, and only better developed in the cercaria and set off from the body as a secondary aquatic adaptation. The excretory epithelium is believed to immobilize wastes and thus permit encysted metacercariae to grow and develop, even to functional sexual maturity as seen in progenetic species. The resemblance of rediae to cercariae in the Bivesiculidae, the ability of certain cyathocotylid sporocysts to produce miracidia, and the recent discovery of gymnophalline cercariae and metacercariae that revert to the function of germinal sacs demonstrate that the trematode life history is a series of polymorphic but fundamentally equivalent generations. Reproduction in germinal sacs is considered to be primarily a type of parthenogenesis in which meiosis is not involved; secondarily, polyembryony may be operative if an embryo that arises parthenogenetically dissociates to produce more than one offspring. In some species, at least, the type and sequence of generations in the molluscan host are not fixed, but may depend on temperature, the extent to which stages infective to that host differentiate before entering it, or the intimacy of the host-parasite relationship. The title of this address is an obvious pun on the familiar quotation from plays attributed to Shakespeare; it was put in the mouth of the clown to provide a bit of ribald comic relief in the tragedy of Othello (Act 3, Scene 1). During this year of celebrating the quadricentennial of Shakespeare's birth, the title, then, is timely and seems doubly appropriate for an address that is largely a tale about tails. Widespread and diversified as caudal appendages are in the animal realm, in no other group do they exhibit the varied development, form, and function seen in the larval trematodes whose name, cercariae, suggests, as is sometimes the case, that the tail does indeed Received for publication 15 September 1964. * Presidential Address, American Society of Parasitologists, Thursday, 27 August 1964 [11:00 AM, Room 140, Chemistry Building], University of Colorado, Boulder. wag the dog. This is particularly true of the large and often pigmented cercariae of the azygiids and bivesiculids whose tails are excellent fish lures and may be nutritious, but their bodies linger on. At the other extreme are the microcercous or even tail-less larvae which are deprived of the ability to swim, but not necessarily that of escaping from the mollusk to seek the next host in the cycle. Between these extremes are the many types of cercariae whose tails are primarily natatory, with swimming facilitated by a variety of modifications responsible for several descriptive group names. Other modifications enable cercariae to float, unite in clusters, or retract the body into the tail. Most remarkable of all are the cystophorous hemiurid cercariae in which the body is withdrawn into a tail that has relinquished swimming ability to become a complex exploding device that conceivably