Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/687

CRUSTACEA 653 still continues to moult probably through its entire lifetime, even to extreme old age. 1 In casting its-shell a crab not only parts with every joint and plate of its limbs and carapace, of its long and slender antennae, its external eyestalks, the plates of the tail, the appendages of the mouth, the lining of its gills, but even itd stomach, with the gastric teeth and the slender apode- mata, which give support to the muscles of the limbs and body so that when the crab has escaped from its old suit, the cast-off shell seems nearly as perfect as the animal itself When exuviating, the crab and lobster both escape from their old shells by a line of dehiscence which opens between the posterior border of the carapace and its union with tho abdomen. Professor Bell also states that in the great crab (Cancer pagurus} and some other forms, the carapace divides at the junction of the epimera with the dorsal piece or tergum. In Limulus the carapace splits all round the anterior border, at the union of the dorsal and ventral walls. Limulus sheds its shell five to six times during the firbt year, and probably once annually after that period. Sir John Dalyell, Mr Couch, Mr Gosse, Mr Spence Bate, Mr Warrington, and others, have given excellent accounts of the process of moulting of various Crustacea. An accu rate observer, Mr Harper, states 2 that he confined six small specimens of the common shore-crab (Carcimis maenas) in separate glasses, and fed them daily, until one of them ehowed that something was amiss by refusing food. Soon after it cast its shell, an operation which only occupied five minutes. When very young this crab moults frequently, The same author registered the dates, and preserved the exuviae of one which moulted on April 11, 1858, and on May 22, July 3, August 30, and September 2G of the same year; the acceleration of the last moult is attributed to the creature having been fed daily, " like a prize beast, " on pur pose to try the effect on its growth. Some of these little crabs had lost part of their limbs, but after a moult new limbs appeared of very diminutive size; after a second moult each new limb had increased to one-half as large as the rest, and in the third moult it had reached to its proper bulk and form. Hermit-crabs shed their hard shell before pulling off the exuviae of the tail ; their increase at each moult is much less rapid than in the common crab. Prawns exu viate more frequently. Mr Warrington saw the change occur with much regularity every twelve days in the summer season. With the exception then of certain parasitic forms of Crustacea, which, like the Rhizocephala, have undergone such a complete retrograde metamorphism that no trace of articulations or appendages remain, all the Crustacea periodically exuviate their dermal covering, whether calcareous, chitinous, or membranaceous. In the Cirripedia it would not be possible to exuviate the adherent shell of the adult Dalanus, or the peduncle of Lepas or ) but, even in this aberrant division, the lining 1 Prof. Bell observes, " There is no doubt that exuviation in many of the higher forms takes place annually with great regularity until tho growth is completed, which in many species is not the case before the animal is many years old. This is proved by the extent to which tho size increases at each moult, compared with the difference between the young and the old animal ; and it is evident that after the growth has reached its maximum, the crust ceases to bo changed, from the fact which I have seen in several instances, as in the common crab, tho lobster, and some others, where the carapace of the still living creature was the seat of barnacles so large, that several years must probably have been required for attaining their existing size " (British Stalk- ei/ed Crustacea, Introduction, xxxiv.). The young male of Limulus, according to Packard, does not attain to the period of puberty before it is four years old. Many Entomostraca infested with bell-animalcules depend on the moulting of their carapace as their only chance of sur viving and escaping these prolific parasites. - Glimpses of Ocean Life (1860). membrane of the shell and the many-jointed cirri are regularly moulted. 3 It has long been known that Crustacea possess the power of voluntarily casting their limbs, and of restoring such as have thus been lost by the animal s will or by accident. 4 If one or more distant phalanges of a limb bo torn off, the animal has the power to throw off the remain ing part of the limb also. This separation always occurs near the basal extremity of the first phalanx. When tho limb is thrown off, the blood-vessels and nerve retract, thus leaving a small cavity ; from this the germ of tho future leg springs, and is at first seen as a nucleated cell. A cicatrix forms over the raw surface caused by the separa tion, which afterwards forms a sheath fur the young le^. 5 PRINCIPAL DIVISIONS OF THE CRUSTACEA. The subjoined table is intended to give only a general outline of the Crustacean class, with the sub-classes, legions, and orders. Of the thirteen orders enumerated two only (printed in italics in the table) are extinct, namely the Trilobita and Eurypterida. These two lost orders disappeared in tha Carboniferous epoch. Table of Classification of the Crustacea* CLASS CRUSTACEA. Sub-Class 1. THORACIPODA (or Halacostraca). Legion I. PODOPUTHALMIA. Order 1. DECAFODA. Sub-Order () Brachyura, Crabs. (b) Anomoura, Hermit-crabs. (c) Macroura, Lobster, Prawn. ,, 2. STOMAPODA, Syuilla, Mysis, Diastylidoc. Legion II. EDRIOPHTHALMIA. ,, 3. ISOPODA, Oniscus, Idotea, Sphccroma. ,, 4. TRILOBITA, Phacops, Asaplius, Calymcne, <Lc ,, 5. AMFIIIPOPA, Talilrus, Gammarus, <L-c. Sub-Class 2. GNA THOPODA (or Ent&mostraea). Legion III. MEROSTOMATA. ,, 6, XIPHOSITRA, Limulus, Bcllinurus, <L-c. ,, 7. EVRYPTEKIDA, Eurijptcrus, Pterygotus. Legion IV. BRANCHIOPODA ,, 8. PHYLLOPODA, Apus,Nebalia,Artcmia. ,, 9. CLADOCERA, Daphnia, Lynceus, d-c. Legion V. LOPHYROPODA. ,, 10. OSTRACODA, Cypris, Candona, Cy there. ,, 11 COPEPODA () Liberata : Cyclops, Cetochilus, Diapt&mus. (1) Parasita : Lcrnanthropus, Caligus, Nicothoe. Legion VI. ANCHORACEPHALA. ,, 12. RHIZOCEPHALA, Sacculina, Pdtogastcr. ,, 13. CauiiPEDlA, (o) Balanida; &.c. (?>) Lepadidcc, &c. If the old definition between the two great groups, tha MALACOSTRACA and the ENTOMOSTRACA, be maintained, namely, that the former shall consist only of forms having 3 The frequency with which they exuviate, together with the dura bility of the cast-off integuments, explains the astonishing masses of exuviae which Mr C. W.Peacli observed annually off the coast of Cornwall, especially in the months of April and May ; but he has seen quantities also in September. He could easily have filled several quart-measui ea with them (Darwin s Balanidae, p. 157). In connection with the exuviation of the Cirriptdia Darwin mention a most remarkable fact (op. cit. p. 15): "In regard to the femali organs, the ovarian tubes and caeca inosculate together ; there are i..j oviducts ; the ova, connected together by membrane, and so forming the oyigerous lamella;, become exposed ly the exuviations of the lining tunic of the carapace or sack, and by the formation of a nc-.v tunic on the underside of these lamellae, a process unknown in other Crustaceans." 4 It is a well-authenticated fact that the roll of thunder and tho discharge of artillery over that part of the sea where lobsters resort will cause them to throw off their great claws. The same effect is alsi> produced by the infliction of any sudden injury. If Porcellann platycheles be seized by the claw .it immediately casts it off and beita a retreat without it.

6 H. Goodsir, Ann. and Mag. Nat. Uist., vol. xiiJ. p. 07.