The Key (Follow option in brackets if feature doesn't apply.)
(18 from fresh water, brackish water, soil or mosses, and an exclusively marine one - distributed in 3 Orders and 5 Families)

1 (32)

Long buccal tube. (Deep Mastax)

2

2 (3)

With complete “corona” (Trochal discs and Cingulum) or at least with reduced trochal discs.

6

3 (2)

Without a “corona”. With a large and flat cephalic ciliated area.

4

Order Adinetida (only one family)

Family Adinetidae

4 (5)

Ribbon-like body, dorso-ventrally compressed. The head, ovoid in dorsal view, is wider than the neck. The ventral zone of the head is oblong with large ciliated cells. The anterior end generally thinner, finishes in a short but complex “rostrum”. A single species with eyes (really ocelli) in the rostrum (A. oculata). But an Australian species (Adineta ricciae) with eyes at the dorsal base of the rostrum has been recently described.  The ventral and posterior end of the “head” has, on its wider part, two lateral cross-sectional laminae (rakes) armed with “teeth” directed towards the front, that scrape the substrate to harvest the food and direct it towards the mouth located among them. Adineta seldom use the creeping movement characteristic of the bdelloids, instead they move extended, gliding very fast on the ciliated cephalic feature. They do not swim. Long pharynx. Evident small foot, with 3 fingers, and 2 spurs. Fourteen species, litoral, or between humid mosses. Uncus with 2 teeth.

Adineta  

5 (4)

Cylindrical or caterpillar shaped. Very short foot, with tiny spurs and papilae (without toes). They don’t swim, crawl. Uncus with 2 teeth. The head has the same diameter as the neck. Two species, that live in soil and mosses.

Bradyscela

Order Philodinida

6 (11)

Stomach without ciliated lumen, syncitial. Plenty of digestive vacuoles, which give it a “frothy aspect”. According to Burger, 1948, the food is formed like pellets in the esophagus, and then included in the vacuoles, which digest them. Feces are pelleted. Narrow “corona”, but separated into 2 distinct trochal discs.

7

Family Habrotrochidae

7 (8)

The pedicels of the “corona” show, at half length, a membranous ring, incomplete, described as flat, and shelf-like, which could be difficult to see and interpret because they rarely extend the trochal discs. Uncus with 6 to 10 teeth. Short foot, without toes, with two small spurs. Seven species, in mosses that dry up, and in the soil, and very few species in the water, between aquatic plants.

Otostephanos

8 (7)

Without the membranous ring in the trochal pedicel.

9

9 (10)

Corona semi-covered by a transparent expansion or lobe of the cuticle, shaped as a pointed hood, and derived from the superior lip. Rarely visible because only expands it when eating. Eyes are not reported. Uncus with 3 to 9 teeth. Short foot, without toes, two small spurs. 9 species that live in mosses or soil.

Scepanotrocha

10 (9)

Without the pointed hood. More than 100 species, in soil, sediments and submerged mosses (but also in mosses that dry up). Long buccal funnel and tube. Very short foot, 2 spurs, 3 fingers. Corona often narrower than the neck. Uncus with up to 10 teeth (always more than 2) of different sizes. Very few species (perhaps only one) have eyes. Many live inside a transparent capsule in bottle form. Some species invade cells of Sphagnum mosses, where they inhabit and deposit their eggs. Others form mucus envelopes with adherences of vegetal detritus. In all the cases the body is differentiated in one anterior part somewhat thinner and with a smoother cuticle and a posterior one with more rigid and slightly carved cuticle.

Habrotrocha

 11 (6)

Glandular stomach, with ciliated lumen, more or less wide. The food doesn’t form pellets, neither in the stomach nor being defecated. Normally the trophi have few teeth.

12

Family Philodinidae

12 (17)

Without toes in the foot, with an adhesive disc.

13

13 (16)

Short foot (smaller or equal to half of the length of the trunk).

14

14 (15)

Only one species, marine. Still now the only strictly marine genus of Bdelloidea. Epizoic,especially on holothuroids, but also on annelids and molluscs. Foot very short, with small spurs and a sticky disc. Each uncus has 2 teeth.

Zelinkiella

15 (14)

Free swimming freshwater species. Oviparous. Short foot without toes. It has a sticky disc, that is sometimes difficult to see. Two small spurs. Eyes are rare. Uncus with sometimes 2, mostly 3, and up to 10 teeth. More or less 50 species. They live on soil and mosses.

Mniobia 

16 (13)

Long foot (longer than half the length of the trunk), finishing in 2 short spurs, and a sticky disc, without toes. Uncus with 2 teeth. Without eyes, with lamellae in the rostrum. Two species, epizoic on freshwater crabs. One of them occurs in Italy on a freshwater crab, and the other in Uruguay, on a brackish water crab on the shores of the “Río de la Plata” (La Plata River).

Anomopus

17 (12)

Foot with more or less visible toes.

18

18 (23)

Viviparous Species. (But see Embata.) Even though they do not have an advanced developed embryo, it is possible to detect the viviparity because the egg in uterus shows more than one nucleus, or is segmented into many small cells; both indications of embryonic development.

19

19 (20)

“Corona” almost always displayed. “Rostrum” (proboscis) and dorsal antenna evident. They feed generally with the rostrum extended. With or without eyes. When they have them, they are located at the end of the proboscis (“rostrum”). Uncus with 2 or 3 teeth. Fine and long foot with 2 long spurs and 3 toes generally very visible (one dorsal and two posterior ones). More or less 20 species, generally litoral, although some are epizoic, or they live in the soil.

Rotaria

20 (19)

The antenna can be seen easily, but they do not have rostrum (proboscis) or this is hardly evident).

21

21 (22)

Smooth epidermis. Long, flat and wide spurs, 4 toes, very long foot (half or full the total length), 5 epizoic species, they are oviparous and ovoviviparous species, One benthic species, E. laticeps frequently inhabits running waters with gravel bottom. Wide trochal disks. Only the viviparous species have eyes. Uncus with 2 teeth.

Embata

22 (21)

Typically ornate cuticle. They can have rigid, high and long thorns, or appendices, in the trunk. With spurs several times longer than the width of the base. Four toes, foot with four segments. Trochal disks rarely visible. Eyes over the brain like in Philodina. Uncus with 2-3 teeth. 7 species, mainly in coastal sediments and submerged moss.

Dissotrocha

23 (18)

Oviparous species.

24

24 (25)

Large, more than 400 µm when creeping. Short foot with just two small blunt toes. Uncus with 4 or 5 teeth. Without eyes. Oviparous, found in mosses and soil. Only one species known.

Didymodactylos

NOTE: This is the original description of the genus as per Milne. (Didymodactilos carnosus Milne, 1916) but some authors spell it as Didymodactylus.

25 (24)

With more than two toes.

26

26 (29)

With three toes.

27

27 (28)

The exterior angles of the cingulum stretch and curve forming 2 long membranous horns or wings. They are extended when eating. Specimens are more or less long, but with a short foot. Uncus with 2-3 teeth. Four species, many times cited as living in acidic water mosses. Ricci and Melone state that they all live in the soil.

Ceratotrocha

28 (27)

Over 100 species. Generally in mosses that dry frequently. Without cuticular expansions in the head. Without eyes. Uncus with 2 teeth, some times 3, rarely 5. Short foot and fingers. Robust aspect. Very marked cuticular segments. Some species with the cuticle of the trunk “carved”, or with thorns. Ample “corona”, generally extended. Some species can be confused at first sight with Rotaria, but they have relatively small spurs and fingers, and they do not have eyes in the proboscis.

Macrotrachela

29 (26)

With four toes.

30

30(31)

They seldom has the trochal discs open. Without eyes. Short foot with quite short spurs (character of fast differentiation with Dissotrocha when they have thorns) and 4 toes. Somewhat rigid cuticle, like a type of armor, sometimes ornamented with thorns. Uncus with 2 teeth. Only one species has 5 teeth. 14 species, all found in submerged mosses, sometimes in acid waters.

Pleuretra  

31 (30)

Body generally extended, although it can be fusiform and even robust. Long foot. Two spurs and 4 toes often visible. Typical “corona” generally extended. Uncus generally with 2 teeth (some species with 5). There is a viviparous species of Philodina, but is easily distinguished from Rotaria because it does not have the eyes in the rostrum. The majority with reddish eyes, placed under the antenna, on the brain and behind the mastax. Most of the species live in water, although there are some in mosses, or in the soil. Approximately 40 species.

Philodina

Although Macrotrachela and Habrotrocha have more than double the species, their habitat makes them less frequent in the samplings of the amateur microscopists. Philodina is probably the more mentioned genus of Bdelloids and its structure (or the one of Rotaria) is the generally chosen one to illustrate the Bdelloidea in handbooks, or articles.

Order Philodinavida

Family Philodinavidae

32 (1)

Mastax superficial, near the mouth. Short buccal tube. The big mastax is protrusible and this allows them to directly scrape the food off the substrate. The inferior lip of the poorly developed corona, forms a 'V' that skirts the buccal opening, which is bilaterally bordered by bent cuticular structures (cheeks), (Ricci and Melone). Stout rostrum, very evident, with a strong ciliated end. Short dorsal antenna.

33

33 (34)

With “corona”, but very reduced and atypical. The upper lip and the lower lip completely lacking. One frontal field, with two areas. The upper one, near the rostrum, only surrounded by cilia (the circumapical waist, De Beauchamp), the lower, and more ventrally oriented (the buccal field, De Beauchamp) covered with much smaller cilia. If the rotifer is seen in ventral or dorsal view the circumapical waist can be distinguished at both sides of the rostrum as small “pseudo-trochas”. They have a stout rostrum, but shorter than in the other two genera. In some pictures by Pourriot (1974) and Ricci et all. (2001) the rostrum is difficult to see between the pseudotrochas. Short foot, Philodinidae style, (approx. 25% of total length) with two spurs and 4 short toes. Two species, one in Europe and North America, that eats cyanobacteria (de Beauchamp, 1909, Pourriot, 1974) and one (carnivore) in the Barbados (Ricci, Melone and Walsh, 2001).

Abrochtha

34 (33)

Without “corona”. Strong, and always extended ciliated proboscis.

35

35 (36)

Long proboscis. Short antenna. A very small pre-oral ciliated area protected by oval cuticular flaps. A very short (3 segmented) foot, with only one, immobile, large spur and four strong toes with form and sizes very similar to the spur. Two species in running water on submerged vegetation.

Henoceros

36 (35)

Cilia are reduced to a small field around the mouth, and those at the end of the always extended short but strong rostrum. Evident short antenna. One of the species (P. aussiensis) has two eyes on the brain. Short foot with 4 strong, often visible toes. Two short, parallel, conical spurs. The existing illustrations show the species with the aspect of caterpillars, due to their cylindrical body, and its short head and very short foot. Two litoral species.

Philodinavus