Body Plan

All arthropods are bilaterally symmetrical; that is, their organs are arranged in pairs such that if the lobster were divided into two equal halves from head to tail, the organ parts would be identical on both sides.

The basic body plan of all lobsters, regardless of infraorder and family, consists of two parts: the cephalothorax which represents the fusion of the head (or cephalon) and the thorax, and the abdomen, which is often misnamed as the tail. Fourteen fused segments (or somites) comprise the cephalothorax and each bears a pair of appendages. (Figure modified from a drawing by: Holthuis, L.B. (1991) Marine Lobsters of the World: An Annotated and Illustrated Catalogue of Species of Interest to Fisheries Known to Date. FAO Species Catalog, FAO Fisheries & Synopsis, No. 125, Vol. 13, FAO-UN, Rome: 292 pps. Used with permission.)

The first segment carries the eyes, which are movable, stalked, and compound; the second bears the antennules (A) which are carried on a three-segmented peduncle (foot). The antennules are bifurcated (divided into two flagella) and carry chemosensory organs. These organs are the functional "nose" of the lobster. The third segment bears the antennae (B) which consists of a five-segmented peduncle and a single flagellum. These are tactile organs. The last three cephalic (four through six) and first three thoracic segments (seven through nine) bear the mouthparts, which are named, starting with those borne upon segment four, the mandibles (or jaws, C), first maxillae (D), second maxillae (E), first maxillipeds (F), second maxillipeds (G), and third maxillipeds (H). The first and second maxillae and first maxillipeds are flat, leaf-like plates used to pass food back to the jaws for crushing and ingestion. The second and third maxillipeds are more leg-like so that they can grip food. The mouth is covered by upper (labrum) and lower (paragnath) lips. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

The remaining segments (ten through fourteen) bear the five pairs of thoracic walking legs, or pereiopods. In the Nephropoidea, the first three legs (A, B, C, D) end in a chela or pincer, with the first chelae (A and B) being the largest (referred to as the claws). These legs are used in food acquisition and bear many taste organs. The fourth (E) and fifth (F) legs lack pincers, ending in a pointed dactyl, and are mainly used for grooming and walking. In females, the opening of the oviducts (through which eggs are released) is located at the base of the third walking legs, while in males, the opening of the sperm duct is located on the base of the fifth walking legs. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

Six segments (somites fifteen through twenty) comprise the abdomen and, unlike those found in the cephalothorax, these segments are not fused, but are connected to each other in such a way as to allow flexibility. The first five segments (fifteen through nineteen) bear paired pleopods or "swimmerets". In the male, the first and second pair of pleopods are modified into copulatory stylets (A and B) which are stiff and grooved for the transfer of a spermatophore (package of sperm). In the female, the first pair of pleopods are reduced, soft and flat (C and D). The remaining pleopods consist of a one-segmented peduncle bearing two flagella. These flagella bear hairs in both the male and the female, but bear longer hairs in mature females in whom they serve as the site of egg attachment (E). The sixth segment (somite 20) bears the tail fan which is divided into a central telson and pairs of uropods on both sides (F and G). These paired uropods are modified pleopods. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

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