In the Crustacea the moot point is whether we consider the paired eyes and first pair of antennae as true appendages belonging to two true segments, or whether they are structures sui generis, not homologous to the other limbs. With regard to the first antennae we are probably safe in assigning them to a true body-segment, since in some of the Entomostraca, e.g. Apus, the nerves which supply them spring, not from the brain as in more highly specialised forms, but from the commissures which pass round the oesophagus to connect the dorsally lying brain to the ventral nerve-cord. The paired eyes are always innervated from the brain, but the brain, or at least part of it, is very probably formed of paired trunk-ganglia which have fused into a common cerebral mass; and the fact that under certain circumstances the stalked eye of Decapods when excised with its peripheral ganglion[[2]] can regenerate in the form of an antenna, is perhaps evidence that the lateral eyes are borne on what were once a pair of true appendages.
Now, with regard to the segmentation of the body, the Crustacea fall into three categories: the Entomostraca, in which the number of segments is indefinite; the Malacostraca, in which we may count nineteen segments, exclusive of the terminal piece or telson and omitting the lateral eyes; and the Leptostraca, including the single recent genus Nebalia, in which the segmentation of head and thorax agrees exactly with that of the Malacostraca, but in the abdomen there are two additional segments.
It has been usually held that the indefinite number of segments characteristic of the Entomostraca, and especially the indefinitely large number of segments characteristic of such Phyllopods as Apus, preserves the ancestral condition from which the definite number found in the Malacostraca has been derived; but recently it has been clearly pointed out by Professor Carpenter[[3]] that the number of segments found in the Malacostraca and Leptostraca corresponds with extraordinary exactitude to the number determined as typical in all the other orders of Arthropoda. This remarkable correspondence (it can hardly be coincidence) seems to point to a common Arthropodan plan of segmentation, lying at the very root of the phyletic tree; and if this is so, we are forced to the conclusion that the Malacostraca have retained the primitive type of segmentation in far greater perfection than the Entomostraca, in some of which many segments have been added, e.g. Phyllopoda, while in others segments have been suppressed, e.g. Cladocera, Ostracoda. It may be objected to this view of the primitive condition of segmentation in the Crustacea that the Trilobites, which for various reasons are regarded as related to the ancestral Crustaceans, exhibit an indefinite and often very high number of segments; but, as Professor Carpenter has pointed out, the oldest and most primitive of Trilobites, such as Olenellus, possessed few segments which increase as we pass from Cambrian to Carboniferous genera.
The following table shows the segmentation of the body in the Malacostraca, as compared with that of Limulus (cf. p. 263), Insecta, the primitive Myriapod Scolopendrella, and Peripatus. It will be seen that the correspondence, though not exact, is very close, especially in the first four columns, the number of segments in Peripatus being very variable in the different species.
| Table showing the Segmentation of various Arthropods | |||||
|---|---|---|---|---|---|
| Malacostraca. | Limulus. | Insecta. | Myriapoda. (Scolopendrella). | Peripatus. | |
| 1 | Eyes | Median eyes | Eyes | ||
| 2 | 1st antennae | Rostrum | Antennae | Feelers | Feelers |
| 3 | 2nd antennae | Chelicerae | Intercalary segment | ||
| 4 | Mandibles | Pedipalpi | Mandibles | Mandibles | Mandibles |
| 5 | 1st maxillae | 1st walking legs | Maxillulae | Maxillulae | 1st jaw-claw |
| 6 | 2nd maxillae | 2nd „ „ | 1st maxillae | 1st maxillae | 2nd jaw-claw |
| 7 | 1st maxillipede | 3rd „ „ | 2nd maxillae | 2nd maxillae | 1st leg |
| 8 | 2nd maxillipede | 4th „ „ | 1st leg | 1st leg | 2nd „ |
| 9 | 3rd maxillipede | Chilaria | 2nd „ | 2nd „ | 3rd „ |
| 10 | 1st ambulatory | Genital operculum | 3rd „ | 3rd „ | 4th „ |
| 11 | 2nd „ | 1st gill-book | 1st abdominal | 4th „ | 5th „ |
| 12 | 3rd „ | 2nd „ | 2nd „ | 6th „ | 6th „ |
| 13 | 4th „ | 3rd „ | 3rd „ | 6th „ | 7th „ |
| 14 | 5th „ | 4th „ | 4th „ | 7th „ | 8th „ |
| 15 | 1st abdominal | 5th „ | 5th „ | 8th „ | 9th „ |
| 16 | 2nd „ | No appendages | 6th „ | 9th „ | 10th „ |
| 17 | 3rd „ | „ | 7th „ | 10th „ | 11th „ |
| 18 | 4th „ | „ | 8th „ | 11th „ | 12th „ |
| 19 | 5th „ | „ | 9th „ | 12th „ | 13th „ |
| 20 | 6th „ | „ | 10th „ | Reduced limbs | 14th „ |
| 21 | [[4]] | „ | Cercopods | [[5]] | |
| Telson | Telson | Telson | Telson | Telson | |
The appendages of the Crustacea exhibit a wonderful variety of structure, but these variations can be reduced to at most two, and possibly to one fundamental plan. In a typical Crustacean, besides the paired eyes, which may be borne on stalks, possibly homologous to highly modified limbs, there are present, first, two pairs of rod-like or filamentous antennae, which in the adult are usually specialised for sensory purposes, but frequently retain their primitive function as locomotory limbs even in the adult, e.g. Ostracoda; while in the Nauplius larva, found in almost all the chief subdivisions of the Crustacea, the two pairs of antennae invariably aid in locomotion, and the base of the second antennae is usually furnished with sharp biting spines which assist mastication. Following the antennae is a pair of mandibles which are fashioned for biting the food or for piercing the prey, and posterior to these are two pairs of maxillae, biting organs more slightly built than the mandibles, whose function it is to lacerate the food and prepare it for the more drastic action of the mandibles. So far, with comparatively few exceptions, the order of specialisation is invariable; but behind the maxillae the trunk-appendages vary greatly both in structure and function in the different groups.
As a general rule, the first or first few thoracic limbs are turned forwards toward the mouth, and are subsidiary to mastication; they are then called maxillipedes; this happens usually in the Malacostraca, but to a much less extent in the Entomostraca; and in any case these appendages immediately behind the maxillae never depart to any great extent from a limb-like structure, and they may graduate insensibly into the ordinary trunk-appendages. The latter show great diversity in the different Crustacean groups, according as the animals lead a natatory, creeping, or parasitic method of life; they may be foliaceous, as in the Branchiopoda, or biramous, as in the swimming thoracic and abdominal appendages of the Mysidae, or simply uniramous, as in the walking legs of the higher Decapoda, and the clinging legs of various parasitic forms.
Without going into detailed deviations of structure, many of which will be described under the headings of special groups, it is clear from the foregoing description and from Fig. [1] (p. [10]), that three main types of appendage can be distinguished: first, the foliaceous or multiramous; second, the biramous; and, third, the uniramous.
We may dismiss the uniramous type with a few words: it is obviously secondarily derived from the biramous type; this can be proved in detail in nearly every case. Thus, the uniramous second antennae of some adult forms are during the Nauplius stage invariably biramous, a condition which is retained in the adult Cladocera. Similarly the uniramous walking legs of many Decapoda pass through a biramous stage during development, the outer branches or exopodites of the limbs being suppressed subsequently, while the primitively biramous condition of the thoracic limbs is retained in the adults of the Schizopoda, which doubtless own a common ancestry with the Decapoda. The only Crustacean limb which appears to be constantly uniramous both in larval and adult life is the first pair of antennae.
We are reduced, therefore, to two types—the foliaceous and biramous. Sir E. Ray Lankester,[[6]] in one of his most incisive morphological essays, has explained how these two types are really fundamentally the same. He compares, for instance, the foliaceous first maxillipede (Fig. [1], A), or the second maxilla (Fig. [1], B) of a Decapod, e.g. Astacus, with the foliaceous thoracic limb of Branchipus (Fig. [1], D), and with the typically biramous first maxillipede of a Schizopod (Fig. [1], F).