Evolution and systematics

The class Cestoda encompasses about 5,100-5,200 species, 680 genera, and 72 families. During 1992-2002, annually 30-40 newly discovered species have been added, mostly recorded from tropical habitats (terrestrial and freshwater) or from marine fishes (especially from sharks and rays).

There is no generally accepted concept on the classification of the tapeworms. As of 1999-2002, the validity of the following 15 orders is widely recognized:

1. Gyrocotylidea. In holocephalan fishes (Chimaeri-formes). Intermediate hosts unknown (life cycle without intermediate hosts was hypothesized). One family (Gyrocotylidae), one to two genera, and about 10 species.

2. Amphilinidea. In freshwater and marine fishes and freshwater turtles. Intermediate hosts: crustaceans. Two families (Amphilinidae and Schizochoeridae), six genera, and about eight species.

3. Caryophyllidea. In siluriform and cypriniform freshwater fishes. Intermediate hosts: tubificid annelids. Four families (Balanotaeniidae, Lytocesti-dae, Caryophyllaeidae, and Capingentidae), about 45 genera, and 140 species.

4. Pseudophyllidea. Mostly in freshwater and marine teleost fishes, also in amphibians, reptiles, birds, and mammals. Intermediate hosts: crustaceans (first or only), usually fishes, rarely other vertebrates (second). Six families (Bothriocephalidae, Philobythiidae, Echinophallidae, Triaenophori-dae, Diphyllobothriidae, and Cephalochlamydi-dae), about 60 genera, and 280 species.

5. Spathebothriidea. In chondrostean and teleost fishes; marine and fresh water. Intermediate hosts: amphipod crustaceans. Two families (Spatheboth-riidae and Acrobothriidae), five genera, and six to seven species.

6. Haplobothriidea. In relict freshwater fishes (Ami-iformes). Intermediate hosts: copepod crustaceans (first) and fishes (second). One family (Hap-lobothriidae), one genus, and two species.

7. Diphyllidea. In elasmobranch fishes. Intermediate hosts unknown. Three families (Echinobothriidae, Macrobothriidae, and Ditrachybothriidae), three genera, and about 35 species.

8. Trypanorhyncha. In elasmobranch fishes. Intermediate hosts: copepod crustaceans, possibly also other marine invertebrates (first), marine teleost fishes (second). Nineteen families (Tentaculariidae, Paranybeliniidae, Hepatoxylidae, Sphyriocephali-dae, Tetrarhynchobothriidae, Eutetrarhynchidae, Gilquiniidae, Shirleyrhynchidae, Otobothriidae, Rhinoptericolidae, Pterobothriidae, Grillotiidae, Molicolidae, Lacistorhynchidae, Dasyrhynchidae, Hornelliellidae, Mustelicolidae, Gymnorhynchi-dae, and Mixodigmatidae), about 50 genera, and 300-350 species.

9. Tetraphyllidea. In elasmobranch and holocepha-lan fishes. First intermediate hosts unknown, larvae found in marine teleost fishes (possible second intermediate or paratenic hosts). Seven families (Cathetocephalidae, Disculicipitidae, Prosoboth-riidae, Dioecotaeniidae, Onchobothriidae, Phyl-lobothriidae, and Chimaerocestidae), about 60 genera, and 800 species.

10. Litobothriidea. In lamniform sharks. Intermediate hosts unknown. One family (Litobothriidae), one genus, and eight species.

11. Lecanicephalidea. In elasmobranch fishes. Intermediate hosts unknown. Four families (Polypocephal-idae, Anteroporidae, Tetragonocephalidae, and Lecanicephalidae), about 12 genera, and 45 species.

12. Proteocephalidea. Mostly in freshwater fishes, also in amphibians and reptiles connected with freshwater habitats. Thaumasioscolex didelphidis, described in 2001 from opossums in Mexico, is the only species of this order known from a mammalian host. Intermediate hosts: copepods (only or first), fishes and amphibians (second). Two families (Proteo-cephalidae and Monticelliidae), about 50 genera, and 320 species.

13. Nippotaeniidea. In freshwater teleost fishes. Intermediate hosts: copepod crustaceans. One family (Nippotaeniidae), two genera, and about six species.

14. Tetrabothriidea. In marine birds and mammals. Intermediate hosts unknown. One family (Tetra-bothriidae), six genera, and about 50 species.

15. Cyclophyllidea. In tetrapods: mostly in birds and mammals, some species in reptiles and amphibians. Intermediate hosts: arthropods, annelids, mollusks or mammals (only or first), fishes, amphibians, reptiles, birds or mammals (second). Eighteen families (Mesocestoididae, Anoplocephalidae, Linstowiidae, Inermicapsiferidae, Thysanosomatidae, Cateno-taeniidae, Nematotaeniidae, Progynotaeniidae, Acoleidae, Dioecocestidae, Amabiliidae, Davainei-dae, Dilepididae, Dipylidiidae, Hymenolepididae, Paruterinidae, Metadilepididae, and Taeniidae), about 380 genera, and 3,100 species.

The phylogenetic relationships and the classification of the tapeworms are often disputed. According to the traditional views, the tapeworms have been considered the most primitive group among the parasitic flatworms. Two subclasses have been recognized within this class: Cestodaria, including the monozoic orders Gyrocotylidea and Amphilinidea, and Eucestoda, comprising remaining orders (mostly polyzoic but also monozoic). Some authorities consider Amphilinidea and Gyrocotylidea as distinct classes within the phylum Platy-helminthes (as Amphilinida and Gyrocotylida, respectively). Sometimes the order Caryophyllidea (encompassing mono-zoic worms) is placed out of the Eucestoda and believed to be close to the amphilinideans and gyrocotylideans.

However, as of 1999-2002, mostly as a result of extensive phylogenetic studies based on morphology (including ultrastructure) and molecular data, a wide consensus has been achieved on several points. The Cestoda, comprising Gyro-cotylidea, Amphilinidea, and Eucestoda, are believed to form a monophyletic and highly derived flatworm group. One of the major characters supporting their monophyly is the lack of an intestine in all stages of their development. Some further characters, mostly connected with ultrastructural peculiarities of the osmoregulatory system and the tissue covering the body, also confirm their origin from a common ancestor.

The tapeworms, together with the monogeneans and the trematodes, belong to a monophyletic taxon named Neoder-mata (i.e., "having new skin"). This name reflects the fact that the ciliated epidermis of the larvae is replaced during the metamorphosis by a peculiar syncytial tissue (tegument or neodermis) occurring in adult worms. Main functions of the tegument are protective (against host's immune reactions and enzymes) and digestive (as a major site of absorption, metabolic transformations, and transport of nutrients). The tegument consists of a surface syncytial layer (distal cytoplasm) connected by cytoplasmic bridges with cell bodies (cytons). The cytons, containing nuclei and possessing powerful secretory apparatus, are situated deeply beneath the superficial muscle layers; therefore, they are well protected against host's reactions. Their secretions permanently renovate the distal cytoplasm, which acts as a contact zone between the parasite body and the host's tissues and fluids. Thus, the tegument is an important adaptation for parasitic life (all the adult neo-dermatans are parasitic). In addition, a range of further characters also is important for defining the Neodermata as a monophyletic group. These are some peculiarities in the structure of the locomotory cilia and the sensory receptors and common patterns of some processes in the course of the spermatogenesis and the formation of the excretory organs.

The monogeneans are believed to be the closest relatives of the tapeworms. The two groups are included in the superior taxon Cercomeromorphae. Their phylogenetic relationships are mostly supported by the presence of set of hooks in the posterior end of the body. In the Eucestoda, these hooks occur in larvae (six embryonic hooks), rarely in both larvae and adults (10 hooks in Gyrocotylidea and Amphilinidea). In eucestode larvae, the embryonic hooks are often situated in a distinct portion of the body (cercomer), which is usually delimited by a constriction from the anterior part of the body. As a rule, the cercomer together with embryonic hooks is detached in the course of the development of the eucestode lar vae before its transmission to the final host. In contrast, the caudal hooks of the monogenean larvae are persistent in adult worms as important elements of their attachment apparatus (haptor). The belief that the eucestode cercomer is homologous to the monogenean haptor (not explicitly supported by recent studies) gave the name of the Cercomeromorphae.

Within the Cestoda, the Gyrocotylidea have a basal position to the branch containing the remaining taxa (Amphilin-idea plus eucestode orders). Among the Eucestoda, the monozoic order Caryophyllidea is considered basal to the remaining orders. Among the polyzoic orders, these having as a rule four suckers or bothridia on the scolex (known as tetrafossate, e.g., Tetraphyllidea, Proteocephalidea, and Cy-clophyllidea) are considered more derived than those having two bothria or bothridia (difossate, e.g., Pseudophyllidea and Diphyllidea).

The monophyly of the Neodermata and the Cercomero-morphae as well as the phylogenetic interrelations within the Cestoda also are well supported by comparisons of their gene sequences.

There are no fossil records of cestodes. However, the phy-logenetic studies and the analyses of the evolutionary associations with hosts suggest a long period of the eucestode-vertebrate coevolution, perhaps since the Devonian (before 350-420 million years).

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