ANNULOSA: CRUSTACEA. 309 



The segments of the abdomen cany each a pair of natatory 

 limbs, or " swimmerets," the last pair being greatly expanded, 

 and constituting, with the " telson," a powerful caudal fin. Most 

 posteriorly of all is the post-anal plate, or " telson," which may 

 be looked upon either as an azygous appendage, or as a ter- 

 minal 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. 137, 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 supporting 

 numerous laminae, and they are richly supplied with blood, 

 but are not ciliated. The water which occupies the gill-cham- 

 bers 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. 



The sexes are invariably distinct, and the generative pro- 

 ducts are conveyed to the exterior by efferent ducts, which 

 open at the base of one of the pairs of thoracic legs. The 

 ovum is " meroblastic," a portion only of the vitellus under- 

 going segmentation. The neural side of the body that is to 

 say, the ventral surface appears on the surface of the ovum, 

 so that the embryo is built up from below, and the umbilicus 

 is situated posteriorly. 



TRIBE B. ANOMURA. The Decapods which belong to this 

 tribe are distinguished by the condition of the abdomen, which 

 is neither so well developed as in the Macrura, nor so nidi- 



