

ANNULOSA: CRUSTACEA. 193 



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 con- 

 tractile sac, which opens by valvular apertures into a sur- 

 rounding venous sinus, inappropriately called the 'pericardium.' 

 The heart is filled with oxygenated blood derived from the 

 gills, and propels the aerated blood through every part of the 

 body. The gills 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 laminaa, 

 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 maxillee, which constantly 

 spoons out the water from the front of the branchial chamber, 

 and thus causes an entry of fresh water by the posterior aper- 

 ture 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 antenna, 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 undergoing 

 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 

 rudimentary as in Crabs. Further, the abdomen does not 

 terminate posteriorly in a caudal fin, as in the Lobster. 



The most familiar of the Anomura are the Hermit-crabs 

 (PagurifjUe). In the common Hermit-crab (Pagurus Bcrn- 

 hardus) the abdomen is quite soft, and is merely enclosed in a 

 membrane, so that the animal is compelled to protect itself by 

 adopting the empty shell of some Mollusc, such as the common 

 Whelk, which it changes at will, when too small. The Hermit 

 is provided with a terminal caudal sucker, and with two or 

 three pairs of rudimentary feet developed upon the abdomen, 

 by means of which he retains his position within his borrowed 



VOL. i. 



