Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 4S2 CRUSTACEA. six pairs of nerves to the antennae and legs take their origin; a siibcespphageal ganglionic mass with three transverse commissures; and a double ganglionic cord, which gives oft* branches to the ventral 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
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 4S2 CRUSTACEA. six pairs of nerves to the antennae and legs take their origin; a siibcespphageal ganglionic mass with three transverse commissures; and a double ganglionic cord, which gives oft* branches to the ventral 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 by eight pairs of slits, which can be closed with valves; it is also provided with arteries, which, after a short course, pass into lacunar blood paths. From the base of the gills, two spaces, returning the blood, extend to the pericardia! sinus. Five pairs of appendages of the abdominal feet function as gills. These are composed of a very large number of delicate lamella3, lying one on another like the leaves of a book. Generative organs.—The branched ovaries unite to foflm two oviducts, 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 the male, the anterior thoracic feet end in simple claws. Development.—It is known that the young leave the egg without the caudal spine and often without the three posterior pairs of gill- 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 abd
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