Quiz 20: Crustaceans
The appendages found on the head of a crustacean are as follows: • Antennule (1 st appendage) • Antennae (2 nd appendage) • Mandible (Jaw) • Maxillae (Accessory jaws) Hence , there are four appendages in the head region of the crustaceans. The characteristic that differ the crustaceans from other arthropods is the presence of two pairs of antennae. Apart from this, crustaceans also have a pair of mandibles and two pairs of maxillae in the head region which are followed by the other appendages in the thorax and abdomen region of the crustaceans. Hence, two pairs of antennae is the distinguishing feature of crustaceans.
The definitions of the following terms are given below: • Tergum- The cuticular plate present on the dorsal surface where the segments are not covered by carapace is called Tergum. • Sternum- The ventral transverse bar which lies between the appendages is called sternum. • Telson- Non segmented part present in the posterior end of the animal is called telson. • Protopod- the basal portion of the appendage which consists of coxa and basis is called Protopod. • Exopod- the lateral of the two branches arising from the Protopod is known as Exopod. • Endopod- the Medial of the two branches arising from the Protopod is called Endopod. • Epipod- A lateral extension (Exite) on the Protopod is called an Epipod. • Endite- Median outgrowth arising from the Protopod of an appendage is called Endite. • Exite-Lateral outgrowth arising from the Protopod of an appendage is called Exite.
The structures that share a similar basic plan and have descended from a common ancestral form which may perform same or different functions are called homologous structures. When several appendages have all developed from a common biramous appendage, these appendages are all homologous to each other. This condition is known as serial homology. Crustaceans have 17 distinct but serially homologous types of appendages. Appendages in crustaceans have become specialized by evolving into a wide variety of walking legs, mouthparts, swimmerets, etc. from modification of the basic biramous appendage. Therefore crustaceans are known to show serial homology.