482 



CRUSTACEA. 



six pairs of nerves to the antenna? and legs take their origin ; a 

 suboesophageal ganglionic mass with three transverse commissures ; 

 and a double ganglionic cord, which gives off 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 lamella 1 , lying one 

 on another like the leaves of a book. 



Generative organs. -The branched 

 ovaries unite to form two oviducts, 

 which open by separate openings 011 

 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 abdomen becomes less obvious (the caudal shield 

 becomes consolidated) and the caudal spine developed. 



The adult animals reach a length of several feet, and live 

 exclusively in the warm seas, in the Indian Archipelago and on the 

 east coast of America. They exist at a depth of two to six fathoms 

 and move about in the mud by the alternating bending and 

 straightening of the cephalic and abdominal shields and the caudal 

 spine. Their food consists chiefly of Nereids. They are found in a 



FIG. 375. Embryo of Limiting in the 

 Trilobite stage (after A. Dohrn). 



