*3.3.1 Valvatidae*

Small wide-spired operculate snails, commonly referred to as valve snails. They are egg-laying and hermaphroditic [87]. Burch [88] lists 11 North American species. According to Strong et al. [89] there are 60 species in the Palaearctic region, 10 in the Nearctic, and 1 for the Afrotropical region. They have a featherlike gill, visible on the left side outside the shell when the snail is active, and a ciliated pallial tentacle extending out to the right.

*Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

#### *3.3.2 Lymnaeidae*

Lymnaeidae (**Figure 5**) is a large and diverse family of freshwater pulmonates widely distributed on all continents except Antarctica. Lymnaeidae exhibit a great diversity in shell morphology which is linked to substantial eco-phenotypic plasticity [90]. Conchological and anatomical traits cannot be taken as reliable diagnostic characters to discriminate species of Lymnaeidae as they vary largely within species [91]. At the supraspecific (genus, subgenus) level there is confusion [92], with some researchers considering numerous genera and subgenera and others only accepting the large genus *Lymnaea*, following the old classification of Hubendick [93]. Phylogenetic analyses, however, show the presence of four well defined subgenera among the genus *Lymnaea* sensu lato, *Radix*, *Galba*, *Leptolimnaea*, and *Lymnaea* [94]. For further details refer to Vinarski et al. [94].

The family is of great parasitological importance as it includes several intermediate hosts of trematodes which infect man and mammals e.g., *Fasciola* spp., and schistosomes infecting both domestic livestock, wild mammals, and birds, the cercariae of all of which can cause swimmer's itch or cercarial dermatitis.

#### *3.3.3 Physidae*

The Physidae has a Holarctic distribution, extending into Central and South America [95]. Physids have been introduced around the world and are common, particularly in lentic habitats. Physid diversity is centered in North America, where they are the most abundant and widespread freshwater gastropods [88]. Physidae are hermaphrodites and can be distinguished from other pulmonates by a high-spired sinistral shell, radula with teeth in V-shaped rows, simple jaw with no lateral processes, and lack of both hemoglobin and a pseudobranch [29]. Other unique characteristics of many species of Physidae are an extended mantle edge that can partly cover the shell, as well as the presence of a preputial gland [29]. Six major clades were uncovered in an analysis of the penial morphology [96], while four major clades, *Physa acuta*, *Physa gyrina*, *Physa fontinalis*, and *Physa pomilia* were recognized by Pip & Franck [97]. They are common hosts of avian schistosomes responsible for causing swimmer's itch. *P. acuta* which is thought to be native to North America has spread throughout the world, and this may pose some difficulties for snail identification (e.g., with confusion with *Bulinus* species) and it may cause displacement of native, disease-transmitting species.

#### *3.3.4 Burnupiidae*

The monogeneric Burnupiidae are a limpetlike group of freshwater pulmonate snails predominantly occurring in Africa. The genus *Burnupia* has traditionally been seen as member of either a freshwater limpet family Ancylidae or as a member of the Planorbidae [98]. The majority of species of *Burnupia* occur in sub-Saharan Africa, particularly in eastern and southern parts, from the isolated Ethiopian highlands down to the Cape region [98].

#### *3.3.5 Bulinidae*

Bulinidae (**Figures 4** and **5**) comprise small to medium-sized planorboid gastropods, reaching up to 25 mm in height or diameter. They are sinistral and either highspired (e.g. *Bulinus*) or discoid (e.g. *Indoplanorbis*) and possess a large pseudobranch that is deeply folded and vascularized [99]. Buliniforme pulmonate gastropods have traditionally been a subfamily of the Planorbidae. Recent molecular phylogenetic analyses, however, have suggested a very different scenario for planorboid gastropods, in which the bulinine forms would be reduced to be represented by *Bulinus* and *Indoplanorbis* only [100, 101]. These phylogenetic suggestions were followed in the most recent classification of the worldwide gastropods [47], in which the family Bulinidae is proposed, comprising the subfamilies Bulininae and Plesiophysinae.

The classification still largely relies on the early accounts of Mandahl-Barth [102, 103], and the system is based on both shell and anatomical characters; however, the definition of the majority of the more than 30 species currently recognized is still unsatisfactory [104]. A variety of taxonomic characters have been employed in *Bulinus*, ranging from (shell) morphology to genital anatomy and radulae, chromosome numbers, and data from electrophoresis and immunodiffusion [104]. Conchological characters are of restricted value in a planorboid snail genus such as *Bulinus* that is characterized by a rather uniform shell shape largely lacking specific characters such as keels [99]. The four species groups have been basically confirmed by phylogenetic studies based on mitochondrial and nuclear markers, but all, unfortunately, suffer from unresolved or poorly supported relationships within and between the proposed species groups [99]. Clearly, many more genetic studies are needed to identify cryptic species (complexes) and to study the role of hybridization of *Bulinus* spp. [99].

*Indoplanorbis exustus* is an intermediate host for the *Schistosoma indicum* species group, and the role of this snail in the transmission of several other medically and veterinary important parasites has been emphasized repeatedly [99]. The species is rather ecologically flexible and thrives in unspecific freshwater habitats that are not flowing, but it requires warm climates. The species is found in Africa, and it is widespread in southern Asia.

#### *3.3.6 Planorbidae*

Planorbidae (**Figures 3** and **6**) represent the most diverse taxon of freshwater pulmonate gastropods on earth that has an almost cosmopolitan distribution [105]. After excluding the Bulinidae and Burnupiidae there are approximately 150 species globally [105]. Following the most recent classification of freshwater gastropods [47], based on various phylogenetic analyses conducted during the past two decades, the Planorbidae consist of three subfamilies, namely Planorbinae Rafinesque, 1815, Ancylinae Rafinesque, 1815, and Miratestinae P. Sarasin & F. Sarasin, 1897 [105].

Planorbidae occur in all kinds of freshwater habitats, ranging from temporary and permanent ponds, streams, rivers, and large lakes [89]. The cosmopolitan distribution of Planorbidae has been the result of a high dispersal capacity and ecological flexibility, including desiccation resistance that is particularly important for the successful passive transport via (aerial) vectors.

The snails are small to medium-sized with long slender tentacles and blood containing hemoglobin [106]. The shell is discoid, lens-shaped, or higher ovate to turreted and the animals are sinistral, that is, the genital openings and the anus are situated on the left side, but in most of the discoid forms the shell appears to be dextral, because it is carried inverted, so that the side representing the spire (apical side) in other families is the lower side of the planorbid shell and the upper side is umbilical [106].

In the Planorbinae, there are several tribes, i.e., Planorbini (almost global distribution); Segmentinini (comprise Palearctic, Oriental, and Afrotropical species); Drepanotrematini (Central and South America); Neoplanorbini (represent

#### *Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

a likely extinct taxon endemic to river systems in the southeasten United States); Helisomatini (includes Afrotropical and American taxa); Coretini (primarily European); and Camptoceratini (southern and eastern Asia) (see references in [105]). Several species are intermediate hosts for medically or veterinary important trematodes including schistosomes.

Freshwater limpets of the subfamily Ancylinae occur on all continents. They are small species with cap- or shield-shaped shell [29]. These animals have a pallial lung, as do all pulmonate snails, but they also have a pseudobranch which serve as a gill in situations where the limpet is unable to reach the surface for air.

The subfamily Miratestinae comprises Australian high-spired planorbid species the buliniform species *Amerianna carinata* that spread widely from its Australian origin [107]. Other species of planorbid snails are global invaders as well [105].
