The following points highlight the twenty important types of Crustaceans. The types are: 1. Hutchinsoniella 2. Branchipus 3. Artemia 4. Daphnia 5. Triops 6. Cypris 7. Derocheilocaris 8. Cyclops 9. Lepas 10. Balanus 11. Mysis 12. Oniscus 13. Homartis 14. Palinurus 15. Scyllarus 16. Eupagurus 17. Hippa 18. Gammarus 19. Caprella 20. Squilla.
Crustaceans: Type # 1. Hutchinsoniella:
It was first collected in the year 1955 from the beach of Long Island Sound in U.S.A. (Fig. 18.22). Length is 3 mm and its characters are of primitive nature. Eyes are absent. Thorax includes eight segments and each segment carries a pair of legs.
All these thoracic appendages and the second maxilla carry gnathobases or endites. Abdominal segments are ten in number and are cylindrical. Abdominal appendages are absent but each segment has pleural spines. Sexes are united (hermaphrodite) but the reproductive organs are different.
Crustaceans: Type # 2. Branchipus:
The size is approximately 15-20 cm. This fresh-water, pink-coloured crustaceans are without carapace. Antennules are short but the antennae are long and modified as prehensile organ.
Thoracic appendages are flattened and used for swimming. Each thoracic appendage bears gill for respiration. A shell gland is present in the cephalic region. Abdomen is distinctly segmented but without appendages. Caudal style is un-jointed.
Crustaceans: Type # 3. Artemia:
It is also known as brine shrimp (Fig. 18.23A). It is noted for its ability to swim on its back. It can tolerate salinity for a considerable period and alteration of salinity produces changes in its structural features. The carapace is absent and the eyes are placed on un-jointed stalks. Abdomen usually consists of six segments. Eleven pairs of thoracic appendages are flattened to facilitate swimming.
While moving during locomotion these limbs-filter the food particles. Respiration takes place through the general surface of the body and through parts of the appendages. Heart is tubular and provided with several pairs of ostia. Excretory organs include one-shell gland which communicates through the base of second maxilla. Larva passes through nauplius stage.
Crustaceans: Type # 4. Daphnia:
Common in fresh-water ponds and pools. The body is laterally compressed and the segmentation is not well marked. A bivalve shell covers the entire body. Prominent biramous antenna and four to six pairs of thoracic legs are used in swimming. Females bear brood pouch.
Crustaceans: Type # 5. Triops:
In older books it is known as Apus. It is a fresh-water form (Fig. 18.23D). Length is 20-30 mm. Anterior two-thirds are enclosed dorsally by carapace. Head is un-segmented, broad and depressed. Abdomen is slender and with distinct segments. A pair of large many-jointed caudal styles extends from last abdominal segment where anus is present.
One pair of lateral eyes and a single median eye are placed on the antero- dorsal side of the carapace. An oval structure, called dorsal organ, is situated posterior to eyes. On each side of the carapace lies a coiled tubular shell gland for excretion. First thoracic appendage is provided with several endites.
Crustaceans: Type # 6. Cypris:
These crustaceans are inhabitants of fresh-water and length varies from 1-2 mm (Fig. 18.23B). Un-segmented body is fully covered by a bivalve carapace, the two halves of which are articulated in the mid-dorsal line and inter-connected by a strong adductor muscle. Carapace may be with varied markings. Compound eyes are absent but a simple median eye is present at the anterior end.
Appendages are seven pairs. Two pairs of antennae are large and work as swimming organs. First maxilla carries a distinct plate and the second maxilla is leg-like. Thoracic appendages are two pairs—first pair is used as leg and the second pair is meant for cleaning.
Abdomen is without appendages. Telson is well formed and bears a pair of small caudal styles. Heart is absent. In an allied species, Pontocypris, the males having 0.3 mm length produce sperm cells which are as long as six millimetres.
Crustaceans: Type # 7. Derocheilocaris:
It was first obtained in the year 1943 from Nebraska beach on Cape Cod (U.S.A.). Length of an adult is 0.4 mm. Biramous mandibles are not specialised. Simple median eye is present. Two pairs of excretory organs (antennal and maxillary) are present.
Crustaceans: Type # 8. Cyclops:
These small elongated and freshwater forms bear distinct segments (Fig. 18.23C). Movement is rapid and jerky. Females possess egg sacs. Cephalothorax is formed by the fusion of head and first thoracic segment. The carapace projects anteriorly as small rostrum. Remaining five thoracic and four abdominal segments are distinctly visible.
A median eye is present on the dorsal side and at the base of the rostrum. Antennules are long and slender. In males, the antennules bend to form a hook for clasping the female during reproduction.
Last thoracic segment carries the genital aperture. The first abdominal segment is fused with the last thoracic segment in females. The last abdominal segment bears anus on the dorsal side. Caudal styles are branched and prominent.
Crustaceans: Type # 9. Lepas:
It is commonly known as ‘goose barnacle’ (Fig. 18.24). The sessile crustacean has a stalk or peduncle on which broad part of the body called the capitulum is placed. The capitulum is covered by carapace or mantle.
The carapace is bivalved and consists of five calcareous plates:
(i) Dorsal carina,
(ii) Two lower and lateral scuta and
(iii) Two lateral and dorsal terga.
The small vermiform body remains within the shell. In a living specimen, the exo- and endopodites of the six pairs of thoracic appendages peep through the partially opened ventral aperture of the mantle and are engaged in food catching.
Antennules are small, but antennae are absent. Two pairs of maxillae are present near the ventral and anteriorly directed mouth. Anterior end of the body is attached with the stalk and the free posterior end terminates as the penis.
Crustaceans: Type # 10. Balanus:
These are called rock barnacles or acorn-shells (18.24). Stalk is absent in these sessile forms and the body is wrapped by a mantle which is made up of six calcareous pieces. The free end is provided with a lid or operculum which is formed by two plates, scuta and terga.
Six pairs of thoracic appendages extend their delicate processes through the gap beneath the carapace. Strong adductor muscles are present for the movement of carapace. Alimentary canal is ‘U’- shaped. Heart and blood vessels are absent. Excretory organs in the adult are called maxillary glands.
Crustaceans: Type # 11. Mysis:
It is almost like a miniature prawn (Fig. 18.23E). The carapace does not extend up to the last few thoracic segments. Thoracic appendages are uniformly leg-like and carry the exopodites. A brood pouch is present near the posterior thoracic append ages. Pleopods are larger in male than in female. Gills are absent. Eyes are larger in comparison to the body size. Statocysts are distinctly visible at the base of the uropods.
Crustaceans: Type # 12. Oniscus:
It is commonly known as wood- louse or saw-bugs. These crustaceans are adapted for land life. Aerial respiration is carried by numerous air-filled tubes. These are small, broad, flattened and more or less oval. Only first thoracic segment participates in the formation of cephalothorax. Compound eyes are sessile. A brood pouch is present.
Crustaceans: Type # 13. Homartis:
It is commonly known as lobster. The body is more or less cylindrical and the chelate legs exhibit symmetry. The exopodites of the uropods bear distinct transverse suture. While moulting the old exoskeleton splits longitudinally along a middle line on the dorsal side. These lobsters are well known for their ability to regenerate lost limbs.
Crustaceans: Type # 14. Palinurus:
The is known as marine rock lobster. The second antenna is long. Rostrum is poorly developed. Legs are not chelated but only clawed. Abdominal appendages are fan-like. The larva is called Phyllosoma.
Crustaceans: Type # 15. Scyllarus:
Strongly built broad body is covered with thick spiny exoskeleton (Fig. 18.25). The second antenna is flat and scale-like. The stalked compound eyes are placed in special sockets within the carapace. First pair of pleopods is absent. The telson and uropods are broad and fan-like. The larva is known as Phyllosoma.
Crustaceans: Type # 16. Eupagurus:
The body of Eupagurus or hermit crab is modified for its peculiar mode of life. It always lives within the empty shell of a gastropod (Fig. 18.26A). Cephalothorax is elongated and bears short antennule and long antenna. The soft abdomen remains twisted within the spiral shell of the mollusc.
Excepting the uropod’s, other abdominal appendages on the right side are absent. Uropod is transformed into a hook-like structure for the attachment within the shell of gastropod. The fourth and fifth pairs of legs are much reduced. The remaining appendages extend out of the shell.
First two legs are chelated and the chelate leg on the right side is very large and acts as lid or operculum. When the growth of the body demands, Eupagurus selects a new large shell and deserts the old one.
Crustaceans: Type # 17. Hippa:
It is commonly known as mole- crab (Fig. 18.26B). The body is more or less cylindrical and resembles the true-crabs in having abdomen flexed beneath the cephalothorax. The oval carapace does not extend up to the anterior part, where antennae and small eyes are present. Thoracic and abdominal appendages are modified for the purpose of digging.
Crustaceans: Type # 18. Gammarus:
This laterally flattened freshwater crustacean is commonly called ‘sand flea’ and is well known for its ability to dart (Fig. 18.27A). Cephalothorax is formed by the fusion of head and one thoracic segment.
The carapace is absent. First antenna is narrow. Mandibular palp is three-jointed. Thoracic appendages are seven pairs—first two pairs are partially chelated, next two pairs are pointed anteriorly and the last three pairs are posteriorly pointed.
The base of thoracic leg is plate-like and a gill is attached with each thoracic leg. Abdomen is well-developed and six-segmented. Each of the first three abdominal segments bears a pair of appendages for swimming and producing water current to bathe the gills. The last three pairs of abdominal appendages are posteriorly directed and known as uropod’s. The uropod’s are used for darting.
Crustaceans: Type # 19. Caprella:
These slender and elongated crustacea resemble the stick insect and exhibit’ perfect mimicry (Fig. 18.27B). The cephalothorax is formed by the fusion of head and first two thoracic segments. The second antenna is smaller than the first. The third thoracic limb is the largest.
The fourth and fifth thoracic legs are absent and these segments bear only the gills. Last three segments carry usual types of legs. The abdomen is rudimentary and persists as a small wart at the end of last thoracic segment. Some extent of hermaphroditism is noted in these forms.
Crustaceans: Type # 20. Squilla:
The notable feature of this crustacea is that the abdomen is much longer than cephalothorax (Fig. 18.26C). Carapace does not cover the last three thoracic segments. Rostrum is movable.
Head has two movable segments in the front—one segment carries the stalked eyes and the other bears the triflagellate antennules. First five pairs of thoracic appendages are uniramous and work as maxillipeds. The second pair of maxillipeds is large and spinny. Pleopods carry the gills.