Shortly before oviposition the female flexes the abdomen; the cavity thus formed becomes filled with a transparent viscid fluid which glues the edges of the opposing somites together. The ova pass into the chamber, are impregnated and suspended to the setae fringing the abdominal limbs, to the abdominal sterna and intersegmental membranes by a hardened layer of the cementing fluid. It is possible that the fluid may set the spermatozoa free from the coat that binds them together.
The ectoderm is composed of a single layer of cells columnar or cubical in certain places. In the inner lamella of the branchiostegite, in the abdominal pleura, and the exo- and endo-podite of the last pair of swimmerets, places where the cuticular structures of the two surfaces are near together, the ectoderm cells are enlarged at intervals, and their bases are connected from one to the other surface by bundles of nucleated fibres which appear to be, but are probably not, continuous with the ectoderm cells themselves. A layer of ectoderm cells also intervenes between the attachment of the muscle fibres to their chitinoid, so-called 'tendons,' which are processes in reality of the cuticular structures, and are moulted with them, as has been proved in the case of the tendons of the adductor mandibulae and of the muscle which adducts the dactylopodite of the forceps.
Beneath the ectoderm is a layer of fibrillated connective tissue, processes of which extend inwards, accompanied by large-celled connective tissue. The pigment cells of this layer are stellate, and contain some a yellow, others a red, pigment, together also with groups of quadrate or oblong crystals of a deep blue colour. Vessels are present, and probably nerves, as irritation of the newly formed integument causes movements on the part of the animal. The moults occur in the warm part of the year (May-September). According to Chantran, they take place (in A. nobilis ?) as follows. The young animal is hatched in May-July. It moults once in 10 days' time; then four times at 20-25 days' interval up to September. Three more moults in May-July following complete the first year of life. There are five in the second year, two in the third, and the animal is now becoming adult. The male is sexually mature after the 15th-17th moult, and henceforward undergoes ecdysis twice yearly. The female is sexually mature in the fourth year, and only moults once a year. The process is very probably in abeyance in old Crayfish, but it is not certain at what age they cease to grow.
According to Carbonnier there are 2-3 moults in the first year, and then the process becomes annual.
When the time approaches for a moult, the old integument becomes softer and thinner, and the ectoderm cells lengthen. The gastroliths and hairs are previously formed. The first step, according to Braun, is the formation of minute processes, 2-5 to each cell, which afterwards form (?) the ridges of the areae (see supra). Vitzou did not observe this formation in any Decapod, but states that the inner surface of the cast-off integument is covered by a soft transparent substance. A section taken through the old and new integuments shows the presence of a new and very delicate cuticle, an outer system of continuous lamellae, and an inner system traversed by vertical lines, corresponding to the contours of the subjacent ectoderm cells. Vitzou concludes from this fact, and from the subsequent shortening of the ectoderm cells, that the lamellae are developed by a specialisation, chemical and physical, of the outer ends of these cells. And his opinion is borne out by the fact that in sections taken parallel to the surface from the carapace of Crabs polygonal areae corresponding to the ectoderm cells may always be seen. The outlines are brought more distinctly into view by treatment with silver nitrate.
But in most instances, e.g. Lobster, these areae are obliterated by early fusion of the cell-walls. Tullberg appears also to be of the opinion that a conversion of the outer ends of the cells takes place during the formation of the new carapace (cf. Zoolog. Jahresbericht, 1882, Arthropoda, p. 12). Vitzou has proved the presence of glycogen in the connective tissues preparatory to and during the moult. Certain tendons, the cornea of the eye and the lining membrane with the hairs of the auditory sac, are regenerated at the same time as the rest of the integument. The endophragmal skeleton is broken up previously to being cast off (Mocquard, C. R. xcvi.). The new cuticle of the alimentary canal is formed at a comparatively late period.
The pores in the lamellae are due to the development of hair-like processes from the cell-surfaces. They are very close-set, and subsequently disappear leaving the pores (Braun).
The old integument splits in all Macrura across the back, between the cephalo-thorax and the first abdominal somite. A split is said to occur also along the limbs. The animal lies upon its side and withdraws first the cephalothorax and then the abdomen with their several appendages. In the Crabs a split occurs along a circular suture between the tergal and pleural regions of the cephalothorax. The animal retains the horizontal position, and the abdomen is freed before the cephalothorax (Vitzou).
The appendages in front of the mouth, and those which are modified into mouth-parts must be disarticulated for the purpose of examination.
The eye-stalk has a short basal and a long terminal joint. It was formerly supposed that it represents a limb, and therefore a somite. It is however derived from a process of the procephalic lobes, and the eye is not stalked at its first appearance in those forms of Crustacea in which it is stalked in the adult when they pass through a complete series of developmental forms commencing with a Nauplius or Metanauplius. The series of true appendages appears to commence with the first antennae.
The eye itself surmounts the terminal joint. It has a convex, soft and transparent cornea marked out by faint lines into square facets. The visual structures are arranged in two layers. To each corneal facet corresponds an eye-element, i. e. a crystalline cone and a retinula. The former is derived from cells of the ectoderm of the procephalic lobes, the latter from the supra-oesophageal ganglion (?). The crystalline cone is formed from four crystalline cells. The nuclei of the cells and a small quantity of protoplasm lie immediately under the corneal facet. The outer part of the crystalline cone is less refractile than the inner part, which is long and has the shape of a four-sided pyramid terminated by a long pointed piece which has four projections fitting into the rhabdome of the retinula. The retinula is composed, as in Palaemon, of retinal cells grouped round a rhabdome formed of four square chitinoid rods. Each rod swells out posteriorly, is red in colour, and is marked by alternate light and dark striae which do not correspond in position in adjoining rods. The retinula cells surround the rhabdome and contain black pigment granules. A basement membrane pierced by the nerve fibres, one to each retinula, separates the retinulae from the optic ganglion.