. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 4S2 CKUSTACEA. six pairs of nerves to the aiitennte and legs take their origin ; a subojsophageal ganglionic mass with three ti-ansverse commissures; and a double ganglionic coi-d, which gives off branches to the venti-al feet and ends with a double ganglion in the abdomen. The alimen- tary canal consists of oesophagus, masticatory stomach, and a straight intestine communicating with a liver and opening by the anus, which is placed immediately in front of the base of the caudal spine. The heart is e
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 4S2 CKUSTACEA. six pairs of nerves to the aiitennte and legs take their origin ; a subojsophageal ganglionic mass with three ti-ansverse commissures; and a double ganglionic coi-d, which gives off branches to the venti-al feet and ends with a double ganglion in the abdomen. The alimen- tary canal consists of oesophagus, masticatory stomach, and a straight intestine communicating with a liver and opening by the anus, which is placed immediately in front of the base of the caudal spine. The heart is elongated and tubular, and is pierced b}^ eight pairs of slits, which can be closed with valves; it is also provided with arteries, which, after a shoii course, pass into lacunar blood paths. From the base of the gills, two spaces, returning the blood, extend to the pericardial sinus. Five pairs of appendages of the abdominal feet function as gills. These are composed of a veiy large number of delicate lamellae, lying one on another like the leaves of a book. Generative organs.—The branched ovaries unite to form two o\-iducts, which open by separate openings on the under side of the operculum (first pair of abdominal limbs); in the male the openings of the two seminal ducts are placed in the same position. In »«K?«nntho ^^^g ^^^^ ^j^^ anterior thoracic feet Tnlobite stage (after A. Dohrn). end in simple claws. Development.—It is known that the young leave the egg without the caudal spine and often without the three posterior paii-s of gUl- bearing feet. This stage has been suitably named the Trilobite stage, on account of the resemblance which the larva presents to a Trilobite (fig. 375). On the cephalic shield there is a median keel- like ridge, which is also found on the abdominal segments. The last abdominal segment includes between its lateral portions the short rudiment of the caudal spine. In the next stage the segmen- tation of the abdomen becomes
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
