[Illustration: FIG. 3.--The Separated Somites and Appendages of the Common Lobster (_Homarus gammarus_).
C, carapace covering the cephalothorax. Ab, abdominal somites. T, telson, having the uropods or appendages of the last abdominal somite spread out on either side of it, forming the "tail-fan." l, labrum, or upper lip. m, metastoma, or lower lip. 1, eyes. 2, antennule (the arrow points to the opening of the so-called auditory organ). 3, antenna. 4, mandible. 5, maxillula (or first maxilla). 6, maxilla (second maxilla). 7-9, first, second and third maxillipeds. ex, exopodite. ep, epipodite. g, gill. 10, sixth thoracic limb (second walking-leg) of female. 11, last thoracic limb of male. In 10 and 11 the arrows indicate the genital apertures. 13, sterna of the thoracic somites, from within. 14, third abdominal somite, with appendages or "swimmerets."]
In no Crustacean, however, do all the somites of the body remain distinct. Coalescence, or suppression of segmentation ("lipomerism"), may involve more or less extensive regions. This is especially the case in the anterior part of the body, where, in correlation with the "adaptational shifting of the oral aperture" (see ARTHROPODA), a varying number of somites unite to form the "cephalon" or head. Apart from the possible existence of an ocular somite corresponding to the eyes (the morphological nature of which is discussed below), the smallest number of head-somites so united in any Crustacean is five. Even where a large number of the somites have fused, there is generally a marked change in the character of the appendages after the fifth pair, and since the integumental fold which forms the carapace seems to originate from this point, it is usual to take the fifth somite as the morphological limit of the cephalon throughout the class. It is quite probable, however, that in the primitive ancestors of existing Crustacea a still smaller number of somites formed the head. The three pairs of appendages present in the "nauplius" larva show certain peculiarities of structure and development which seem to place them in a different category from the other limbs, and there is some ground for regarding the three corresponding somites as constituting a "primary cephalon." For practical purposes, however, it is convenient to include the two following somites also as cephalic.