Worms, Platyhelminthes oncomiracidium The more complex structures consisting of multiple sclerites, which are found only in polyopisthocotyleans, are termed clamps Four rhabdomeric ocelli are present in at least one stage of development The mouth is almost always located in the anterior region of the body Most monogeneans possess a conspicuous muscular pharynx and an intestine that is usually divided into two, or rarely one, blind cecum; the caeca are occasionally branched or anastomose posteriorly All monogeneans are hermaphroditic The male and female genital pores are ventral and usually combined The number of testes ranges from one to many among different groups Most groups possess a single ovary that varies somewhat in form from compact to double inverted U-shaped The male intromittent organ can be a penis or a cirrus, but the exact form of this feature is not well known in some groups The terminal genitalia are often complex The excretory system consists of two sets of protonephridia that open separately to the outside, usually in the anterior of the body, but that may be connected at some point in the body Molecular work has called the monophyly of the class Monogenea into question This is reflected in the unresolved nature of this node in the tree presented in Figure However, the mutual monophyly of the subclasses Monopisthocotylea and Polyopisthocotylea appears to be well supported Members of these subclasses are relatively easy to recognize Most conspicuously, the Polyopisthocotylea possess a haptor that is subdivided into multiple suckers or bears distinctive attachment structures known as clamps, and the Monopisthocotylea, possess a haptor that, although it may bear loculi, is undivided and, although it may bear anchors or hamuli, never bears clamps Furthermore, unlike the monopisthocotyleans, the polyopisthocotyleans possess a genitointestinal canal such that their genital ducts open into their intestine Boeger and Kritsky (2001) provide a number of additional features unique to each subclass The interrelationships among the orders in each subclass are controversial The orders in each Subclass are treated below in alphabetical To make the following diagnoses more concise, unless mentioned, it should be assumed that an order possesses the following: a posterior, discoidal, undivided, symmetrical haptor, a conspicuous muscular pharynx, a gut consisting of two blind intestinal ceca, paired testes, biflagellated sperm, flagella incorporated into body of sperm, 9ỵ microtubule arrangement, a single ovary, a vitellarium that is follicular with follicles arranged in two extensive lateral fields, operculate ectolecithal eggs, and a single, ventral genital pore (Plate 2) 447 internal vesicle, or seminal vesicle absent; penis sac present or absent; penis present or absent; penis elongate, muscular, spines absent; some with cirrus; prostatic complex present; vagina present or absent; seminal receptacle present or absent; genital apertures usually combined, marginal; oviparous Habitat: Adults parasitic on skin or gills of marine teleosts (including remoras), or on skin or gills of elasmobranchs Dactylogyridea See Figure 18 (Plate 2) With two or more pairs of head organs, with or without cephalic glands; posterior half of body covered with anteriorly directed cuticular spines or not; haptor with one or two pairs of anchors supported by one to four transverse bars, with or without accessory plaques, occasionally with two suckers and median terminal anchor complex, usually 14 or 16 marginal hooklets; mouth subterminal, ceca fused posteriorly or not, with lateral diverticula or not; testes usually one; sperm with one axoneme; vas deferens encircling left cecum; seminal vesicle usually present; prostatic complex present; male copulatory organ sclerotized, elongated, muscular, ovary compact, lobed or tubular; vitellarium follicular or occasionally divided into tubular lobules in form of frond-like sprays spreading in two or three lateral groups (Protogyrodactylidae), coextensive with intestinal ceca; seminal receptacle and vagina present or absent; oviparous; eggs oval or tetrahedral; genital aperture median or marginal Habitat: Adults parasitic on gills and skin of marine and freshwater, occasionally brackish water, teleosts; rarely on gills or in heart of elasmobranchs Gyrodactylidea See Figure 19 (Plate 2) With two head organs; haptor usually with one pair of anchors often supported by dorsal and ventral transverse bars with 16 marginal hooklets; mouth ventral, intestinal ceca, one or two, usually not fused posteriorly; testis single; sperm microtubules absent; vas deferens encircling left caecum; male copulatory organ sclerotized or muscular, spines present or absent; accessory piece present; vitellarium symmetrical, near posterior part of intestinal cecae; vagina absent or present, opening ventrally near left margin of body; genital pore marginal; most species viviparous; eggs oval or tetrahedral Habitat: Adults parasitic on gills or skin of freshwater and marine teleosts, in stomach and intestine of amphibians, and gills and tentacles of squid; udonellids on marine crustaceans Lagarocotylidea Subclass Monopisthocotylea Capsalidea See Figure 17 (Plate 2) Anterior end with paired petaloid head organs or paired glandular pseudosuckers; haptor sometimes subdivided by septa into central area and multiple peripheral loculi, with 14 marginal hooklets and two or four central anchors and two anterior sclerites; mouth ventral; intestinal ceca fused posteriorly, with lateral and axial diverticula; testes, two to many; sperm microtubules absent; vas deferens forming bipartite external seminal vesicle and simple See Figure 20 (Plate 2) Oral sucker-like organ absent; with cephalic glands; haptor with 14 submarginal and subcentral hooklets; mouth midventral; ceca not fused posteriorly, without lateral diverticula; with single U- or V-shaped testis; vas deferens encircling left caecum; seminal vesicle present; prostatic reservoir dorsal to copulatory complex; male copulatory organ sclerotized, with accessory piece; ovary compact; vitellarium consisting of two dense bilateral bands of follicles; seminal receptacle absent; vaginal pore dextro-ventral at level of genital pore; eggs without filaments; genital aperture median Habitat: Adults parasitic in the cloaca of salamanders