- Though concealed within the bivalved shellcover, the mouth-parts are nearly as in the Gymnophylla, but the flexing of the caudal part is in contrast, and the biramous second antennae correspond with what is only a larval character in the other phyllopods.
(I) first antenna; (6) tergum; (2) compound (7) biramous eye; feet; (3) liver; (8) carina; (4) simple eye; (9) cement (5) scutum; gland.
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.
- Squilla mantis bases tends to show that the primitive (Stomatopoda), showing the type of appendage was more complex last four thoracic (leg-bearthan the simple biramous limb, and ing) somites free from the some authorities have regarded the carapace.
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.
In the Malacostraca, the antennules are often biramous, but there is considerable doubt as to whether the two branches represent the endopodite and exopodite of the other limbs, and three branches are found in the Stomatopoda and in some Caridea.
The mandibles, like the antennae, have, in the nauplius, the form of biramous swimming limbs, with a masticatory process originating from the proximal part of the protopodite.
This form is retained, with little alteration in some adult Copepoda, where the biramous " palp " still aids in locomotion.
In the Cirripedia (Thyrostraca) the six pairs of biramous cirriform limbs differ only slightly from each other, and in many Copepoda this is also the case.
The thoracic limbs have the endopodites converted, as a rule, into more or less efficient walking-legs, and the exopodites are often lost, while the abdominal limbs more generally preserve the biramous form and are, in the more primitive types, natatory.
The antennules are uniramous, the others biramous, and all three pairs are used in swimming.
The limb-buds early become bilobed and grow out into typical biramous appendages which gradually assume the characters found in the adult.
In those Copepods in which the palps of the mandibles as well as the antennae are biramous and natatory, the first three pairs of appendages retain throughout life, with little modification, the shape and function which they have in the nauplius stage, and must, in all likelihood, be regarded as approximating to those of the primitive Crustacea.
The eyes were probably stalked, the antennae and mandibles biramous and natatory, and both armed with masticatory processes.
It is likely that the trunk-limbs were also biramous, with additional endites and exites.