2l6 MANUAL OF ZOOLOGY. 



organs, the larger called the "great antennae" (fig. 71, ga), the 

 smaller the " antennules " (a). The mouth is situated on the 

 under surface of the front of the head, and is provided from 

 before backwards with an upper lip (" labrum "), two " mandi- 

 bles," two pairs of " maxillae," three pairs of " maxillipedes " 

 or "foot-jaws," and a bifid lower lip, or "metastoma." The 

 five remaining segments of the thorax carry the five pairs of 

 ambulatory legs, of which the first (fig. 71, i) constitute the great 

 claws, or " chelae ; " the next two pairs (2 and 3) are also che- 

 late, though much smaller; and the last two pairs are terminated 

 by simply pointed extremities. The segments of the abdomen 

 carry each a pair of natatory limbs, or " swimmerets," the last 

 pair being greatly expanded, and constituting, with the " tel- 

 son," a powerful caudal fin. Most posteriorly of all is the 

 post-anal plate, or " telson," which may be looked upon either 

 as an azygos appendage, or as a terminal segment which has 

 no lateral appendages. 



The mouth leads by a short oesophagus into a globose 

 stomach, in the cardiac portion of which is a calcareous appa 

 ratus for triturating the food, which is commonly called the 

 "lady in the lobster." The intestine is continued backwards 

 from the stomach without convolutions, and the anal aperture 

 is situated just in front of the telson. There is, also, a well- 

 developed liver, consisting of two lobes which open by separate 

 ducts into the intestine. 



The heart is situated dorsally, and consists of a single poly- 

 gonal contractile sac, which opens by valvular apertures into 

 a surrounding venous sinus, inappropriately called the " peri- 

 cardium." The heart is filled with oxygenated blood derived 

 from the gills, and propels the aerated blood through every 

 part of the body. The gills (fig. 59, 3,^) are pyramidal bodies 

 attached to the bases of the legs, and protected by the sides 

 of the carapace. They consist each of a central stem sup- 

 porting numerous laminae, and they are richly supplied with 

 blood, but are not ciliated. The water which occupies the 

 gill-chambers is renovated partly by the movements of the legs, 

 and partly by the expanded epipodite of the second pair of 

 maxillae, which constantly spoons out the water from the front 

 of the branchial chamber, and thus causes an entry of fresh 

 water by the posterior aperture of the cavity. 



The nervous system is of the normal " homogangliate " type, 

 consisting of a longitudinal series of ganglia of different sizes, 

 united by commissural cords, and placed along the ventral 

 surface of the body. The organs of sense consist of the two 

 compound eyes, the two pairs of antennae, and two auditory sacs. 



