THE DECAPODA 285 



in the delicate skin of the abdomen, especially on the dorsal side 

 anteriorly. Two pairs of venous trunks running along the sides 

 of the abdomen return the blood to the pericardium, a pair of 

 rhythmically contractile vesicles at the base of the abdomen serving 

 to accelerate the now. 



Excretory System. In all Decapods the antennal gland is well 

 developed, and generally presents a complexity of structure not 

 found elsewhere among Crustacea. It has in most cases lost its 

 original tubular form and assumed that of a compact gland. Three 

 divisions are commonly distinct the saccule, the labyrinth, and the 

 bladder, with its efferent duct leading to the exterior. The 

 saccule, which represents the end- sac of the typical antennal 

 gland, may retain its simple saccular form, but more commonly 

 it is complicated either by the development of partitions dividing 

 up its cavity, or by numerous branches which ramify through 

 the mass of the labyrinth. The labyrinth may be considered as 

 derived from a sac which, by the rich development of partitions 

 and trabeculae from its walls, has been converted into a spongy 

 mass traversed by a complex system of canals. In the Crayfish 

 (Astacus) the structure is still further complicated, mainly by the 

 elongation of a portion of the labyrinth into a whitish cord of 

 spongy substance which is convoluted upon itself, forming the 

 "medullary" portion of the gland, the greenish "cortical" layer 

 representing the proximal portion of the labyrinth which com- 

 municates with the end-sac. The bladder may retain, as in the 

 Crayfish, the form of a simple vesicle communicating with the 

 exterior by a short duct. In many cases, however, it sends oft' 

 prolongations which may extend through a great part of the body. 

 In some Caridea this vesical system is very extensive, lobes from 

 the two sides uniting with each other to form an unpaired vesicle 

 above the stomach. In the Brachyura three main lobes are given 

 off from the bladder, which are very constant throughout the group, 

 such differences as do occur being correlated with the differences 

 in shape of the carapace. In the Paguridae, however, the vesical 

 system reaches its greatest complexity (Fig. 167). The bladder 

 sends off prolongations which ramify between the organs and 

 anastomose to form delicate networks and arborisations in the 

 region of the thorax, and two long diverticula, which may unite 

 with each other, pass backwards to traverse the whole length of 

 the abdomen. In Palinurus an accessory gland not found in any 

 other type opens into the duct of the bladder. The external 

 aperture is in most cases placed on a papilliform elevation on the 

 proximal segment of the antennal peduncle. In the Brachyura 

 the aperture is covered by an operculum (Fig. 147, B and C, /), 

 capable of being opened and closed by special muscles. It has 

 been shown that this operculum in all probability represents the 



