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.
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| Fig. 4.—Diagram of an Amphipod. (After Spence Bate andWestwood.) | |
C, cephalon. Th, thorax. (Only seven of theeight thoracic somites arevisible, the first being fusedwith the cephalon.) | Ab, abdomen. The numbers appended to thesomites do not correspond to theenumeration adopted in the text.21 is the telson. |
A remarkable feature found only in the Stomatopoda is the reappearance of segmentation in the anterior part of the cephalic region. Whether the movably articulated segments which bear the eye-stalks and the antennules in this aberrant group correspond to the primitive head somites or not, their distinctness is certainly a secondarily acquired character, for it is not found in the larvae, nor in any of the more primitive groups of Malacostraca.
The body proper is usually divisible into two regions to which the names thorax and abdomen are applied. Throughout the whole of the Malacostraca the thorax consists of eight and the abdomen of six somites (fig. 4), and the two regions are sharply distinguished by the character of their appendages. In the various groups of the Entomostraca, on the other hand, the terms thorax and abdomen, though conveniently employed for purposes of systematic description, do not imply any homology with the regions so named in the Malacostraca. Sometimes they are applied, as in the Copepoda, to the limb-bearing and limbless regions of the trunk, while in other cases, as in the Phyllopoda, they denote, respectively, the regions in front of and behind the genital apertures.
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| Fig. 5.—Phyllopoda and Phyllocarida. | |
1, Ceratiocaris papilio, U. Silurian,Lanark. 2, Nebalia bipes(one side ofcarapace removed). 3, Lepidurus Angassi: a, dorsalaspect; b, ventral aspect ofhead showing the labrum andmouth-parts. | 4, larva of Apus cancriformis. 5, Branchipus stagnalis: a, adultfemale; b, first larval stage(Nauplius); c, second larvalstage. 6, Nauplius of Artemia salina. |
A character which recurs in the most diverse groups of the Crustacea, and which is probably to be regarded as a primitive attribute of the class, is the possession of a carapace or shell, arising as a dorsal fold of the integument from the posterior margin of the head-region. In its most primitive form, as seen in the Apodidae (fig. 5, 3) and in Nebalia (fig. 5, 2), this shell-fold remains free from the trunk, which it envelops more or less completely. It may assume the form of a bivalve shell entirely enclosing the body and limbs, as in many Phyllopoda (fig. 6) and in the Ostracoda. In the Cirripedia it forms a fleshy “mantle” strengthened by shelly plates or valves which may assume a very complex structure. In many cases, however, the shell-fold coalesces with some of the succeeding somites. In the Decapoda (fig. 3), this coalescence affects only the dorsal region of the thoracic somites, and the lateral portions of the carapace overhang on each side, enclosing a pair of chambers within which lie the gills. The arrangement is similar in Schizopoda and Stomatopoda (fig. 7), except that the coalescence does not usually involve the posterior thoracic somites, several of which remain free, though they may be overlapped by the carapace.
| From Morse’s Zoology. |
| Fig. 6.—Estheria, sp.; D from Dubuque, Iowa; (e) the eye. L from Lynn, Massachusetts (nat. size). S presents a highly magnified section of one of the valves to show the successive moults. B an enlarged portion of the edge of the shell along the back, showing the overlap of each growth. |
In the Isopoda and Amphipoda, where, as a rule, all the thoracic somites except the first are distinct (fig. 4), there seems at first sight to be no shell-fold. A comparison with the related Tanaidacea (fig. 8) and Cumacea (or Sympoda), however, leads to the conclusion that the coalescence of the first thoracic somite with the cephalon really involves a vestigial shell-fold, and, indeed, traces of this are said to be observed in the embryonic development of some Isopoda. It seems likely that a similar explanation is to be applied to the coalescence of one or two trunk-somites with the head in the Copepoda, and, if this be so, the only Crustacea remaining in which no trace of a shell-fold is found in the adult are the Anostracous Phyllopoda such as Branchipus (fig. 5, 5).
| Fig. 7.—Squilla mantis (Stomatopoda), showing the last four thoracic (leg-bearing) somites free from the carapace. |
General Morphology of Appendages.—Amid the great variety of forms assumed by the appendages of the Crustacea, it is possible to trace, more or less plainly, the modifications of a fundamental type consisting of a peduncle, the protopodite, bearing two branches, the endopodite and exopodite. This simple biramous form is shown in the swimming-feet of the Copepoda and Branchiura, the “cirri” of the Cirripedia, and the abdominal appendages of the Malacostraca (fig. 3, 14). It is also found in the earliest and most primitive form of larva, known as the Nauplius. As a rule the protopodite is composed of two segments, though one may be reduced or suppressed and occasionally three may be present. In many cases, one of the branches, generally the endopodite, is more strongly developed than the other. Thus, in the thoracic limbs of the Malacostraca, the endopodite generally forms a walking-leg while the exopodite becomes a swimming-branch or may disappear altogether. Very often the basal segment of the protopodite bears, on the outer side, a lamellar appendage (more rarely, two), the epipodite, which may function as a gill. In the appendages near the mouth one or both of the protopodal segments may bear inwardly-turned processes, assisting in mastication and known as gnathobases. The frequent occurrence of epipodites and gnathobases tends to show that the primitive type of appendage was more complex than the simple biramous limb, and some authorities have regarded the leaf-like appendages of the Phyllopoda as nearer the original form from which the various modifications found in other groups have been derived. In a Phyllopod such as Apus the limbs of the trunk consist of a flattened, unsegmented or obscurely segmented axis or corm having a series of lobes or processes known as endites and exites on its inner and outer margins respectively. In all the Phyllopoda the number of endites is six, and the proximal one is more or less distinctly specialized as a gnathobase, working against its fellow of the opposite side in seizing food and transferring it to the mouth. The Phyllopoda are the only Crustacea in which distinct and functional gnathobasic processes are found on appendages far removed from the mouth. The two distal endites are regarded as corresponding to the endopodite and exopodite of the higher Crustacea, the axis or corm of the Phyllopod limb representing the protopodite. The number of exites is less constant, but, in Apus, two are present, the proximal branchial in function and the distal forming a stiffer plate which probably aids in swimming. It is not altogether easy to recognize the homologies of the endites and exites even within the order Phyllopoda, and the identification of the two distal endites as corresponding to the endopodite and exopodite of higher Crustacea is not free from difficulty. It is highly probable, however, that the biramous limb is a simplification of a more complex primitive type, to which the Phyllopod limb is a more or less close approximation.