Eumalacostraca

(invertebrate zoology) A series of the class Crustacea comprising shrimplike crustaceans having eight thoracic segments, six abdominal segments, and a telson.

The major subdivision (subclass) of the crustacean class Malacostraca. It comprises three, four, or five Recent superorders: Syncarida, Peracarida, and Eucarida are most commonly included and are all that are recognized here. However, some carcinologists still insist on ranking the Hoplocarida among eumalacostracans. Acceptance of a separate superorder, Pancarida for the Thermobaenacea, has not received general support, and thermobaenaceans are commonly now included with peracaridan taxa.

Formerly, the concept of the Eumalacostraca as a homogeneous evolutionary unit generated virtually no controversy. The subclass was defined by a series of characters referred to as the caridoid facies: carapace enclosing the thorax; stalked, movable eyes; biramous antennules; scalelike antennal exopod; thoracopods with natatory exopods; abdomen with well-developed, complex musculature designed for flexing it; telson and uropods forming a tailfan; biramous pleopods 1–5; and internal organs primarily excluded from the abdomen. Opponents of the caridoid theory point to the fact that several of these attributes are not restricted to eumalacostracans, and conversely, not all eumalacostracans possess all of them. The carapace, its origin and evolution, has provided the greatest controversy.

The basic body plan common to all eumalacostracans includes a head of five somites, a thorax of eight somites, and an abdomen of six somites, plus a terminal telson. Zero to three pairs of thoracic somites may be fused to the head and their thoracopods and modified to form feeding appendages (maxillipeds). The thorax usually has five to eight pairs of ambulatory or specialized appendages (pereopods); the abdomen may or may not have paired appendages (pleopods) on the first five somites; and the sixth somite usually has a pair of well-developed appendages (uropods). Female gonopores (coxal or sternal) are present at the level of the sixth thoracic somite, while male gonopores are at the level of the eighth.

Two caridoid attributes shared by the majority of eumalacostracans are the strong abdominal musculature and well-developed tailfan. Together with accompanying neural elements, these appear to be designed as a highly efficient propulsion mechanism, that is, an escape reaction.

Most eumalacostracans are dioecious; however, some isopods, tanaids, and a few decapods are hermaphroditic. Fertilization is by means of spermatophores and may be external or internal. See also Crustacea; Eucarida; Malacostraca; Peracarida; Syncarida.

Eumalacostraca
Blue crabs, Callinectes sapidus
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Subclass: Eumalacostraca Grobben, 1892
Superorders
Syncarida Peracarida Eucarida See text for orders.

The Eumalacostraca (Greek: "true soft shell") are a subclass of crustaceans, containing almost all living malacostracans, about 22,000 described species. (The outgroups are the Phyllocarida and possibly the Hoplocarida or mantis shrimps.)

Eumalacostracans have 19 segments (5 cephalic, 8 thoracic, 6 abdominal). The thoracic limbs are jointed and used for swimming or walking. The common ancestor is thought to have had a carapace, and most living species possess one, but it has been lost in some subgroups.

Classification

Martin and Davis ^[1] present the following classification of living eumalacostracans into orders, to which extinct orders have been added, indicated by †.

The group as originally described by Karl Grobben ^[2] included the Stomatopoda (mantis shrimp), and some modern experts continue to use this definition. Here we follow Martin and Davis in excluding them; they are placed in their own subclass, Hoplocarida.

Subclass Eumalacostraca Grobben, 1892

• Superorder Syncarida Packard, 1885
  • †Order Palaeocaridacea
  • Order Bathynellacea Chappuis, 1915
  • Order Anaspidacea Calman, 1904 (including Stygocaridacea)
• Superorder Peracarida Calman, 1904
  • Order Spelaeogriphacea Gordon, 1957
  • Order Thermosbaenacea Monod, 1927
  • Order Lophogastrida Sars, 1870
  • Order Mysida Haworth, 1825
  • Order Mictacea Bowman, Garner, Hessler, Iliffe & Sanders, 1985
  • Order Amphipoda Latreille, 1816
  • Order Isopoda Latreille, 1817 (pillbugs, sowbugs, woodlice)
  • Order Tanaidacea Dana, 1849
  • Order Cumacea Krøyer, 1846
• Superorder Eucarida Calman, 1904
  • Order Euphausiacea Dana, 1852
  • Order Amphionidacea Williamson, 1973
  • Order Decapoda Latreille, 1802 (crabs, lobsters, shrimp)

References

Wikispecies has information related to:

Eumalacostraca

1. ^ J. W. Martin & G. E. Davis (2001). An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County. 
2. ^ C. Grobben (1892). Zur Kenntnis des Stammbaumes und des Systems der Crustaceen. Sitzber. K. Akad. Wiss., Vienna, Math. Nat. Cl. 101: 237–274. 

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