. A manual of elementary zoology . Zoology. THE CRAYFISH 249 another, more powerful, set of muscles connects the sterna with one another and with the endophragmal skeleton. These, when they contract, draw closer the sterna and thus bend the abdomen. By this movement, spreading at the same time its tail fan, the crayfish carries out the sudden backward jumps by which it escapes from its enemies. Its gentle forward movements are carried out by the walking legs, aided by a paddling of the abdominal limbs. The legs of the first three pairs Dull and those of the'last pair push, ^^^C^'7"
. A manual of elementary zoology . Zoology. THE CRAYFISH 249 another, more powerful, set of muscles connects the sterna with one another and with the endophragmal skeleton. These, when they contract, draw closer the sterna and thus bend the abdomen. By this movement, spreading at the same time its tail fan, the crayfish carries out the sudden backward jumps by which it escapes from its enemies. Its gentle forward movements are carried out by the walking legs, aided by a paddling of the abdominal limbs. The legs of the first three pairs Dull and those of the'last pair push, ^^^C^'7"" and their movements . jig2SkiUim$^ ./ {9" are carried out in such a way that the animal is always standing upon six legs while two — which are on opposite sides and of different pairs—are in motion. The body of the crayfish c o ntains a spaci- ous peri- visceral cavity, in which the internal organs lie. This is not a ccelom, but an enlarged portion of the hasmoccele (p. 230), and com- municates with the blood vessels. The alimentary canal fills the greater part of this cavity. The mouth is an elongated opening below the head between the mandibles. It is bordered in front by a wide upper lip or labrum, and behind it stands a pair of lobes known together as the lower lip or metastoma. A short, wide gullet leads upwards into the large proventriculus, often called the "; This consists of two chambers, a large forepart or mill-chamber, often known as the "cardiac division of the stomach," and Perivisceral Cavity and Alimentary , Fig. 158.—A semi-diagrammatic drawing of a transverse section of the abdomen of the crayfish. /'/<., Basipodite; cp., coxopodite; , dorsal abdominal artery ; en., endopodite ; ex., exo- podite ; , extensor muscles ', , flexor muscles; , hind-gut; pi., pleuron; pr., protopodite ; tg., tergum ; St., sternum; , ventral abdominal artery; ventral nerve Ple
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
