vi ARACHNOIDEA INTESTINAL CANAL 525 



The dorsal wall of the hind-gut is often bulged out in the form of 

 a muscular sac. It then looks as if the Malpighian vessels on the one 

 hand and the mid-gut on the other entered a common terminal sac, not 

 at its blind end but near the anal aperture. In the structure of its 

 walls this rectal vesicle, which is also often called the cloaca, agrees with 

 the Malpighian vessels, and not with the mid-gut. This favours the 

 view that the excretory tubes of the Arachnoidea, like the Malpighian 

 vessels of insects, are invaginations of the hind-gut, and consequently 

 ectodermal formations. 



Salivary glands have often been described in the Arachnoidea, but our knowledge 

 of them, especially of their manner of emerging, is very inadequate. The glands 

 which open on the pedipalps are also often regarded as salivary glands. In certain 

 Acarina (Oribatidce) a pair of glands emerging at the boundary between the fore-gut 

 and the mid-gut has been observed. In various Arachnoidea there are groups of 

 glands in the upper lip. 



The anatomy of the diverticula of the mid-gut varies greatly in the different 

 orders. In the Scorpionidce (Fig. 368) they form a 5-lobed mass on each side in the 

 pre - abdomen, this mass being connected with the mid-gut by means of 5 canals 

 (hepatic ducts). In the mid-gut of Solpuga (Galeodes) numerous branched diverticula 

 are said to enter both its anterior and its posterior ends. In the Pseudoscorpionidce 

 there are 3 diverticula of the mid-gut, 2 lateral, and 1 unpaired ventral. The two 

 lateral diverticula again subdivide at their outer edges into 8 lobes. The mid-gut 

 here forms a double loop. In the Microthelyphonidce 5 pairs of shallow bulgings have 

 been observed in the mid-gut. In the mid-gut of the Araneidce (Fig. 369, A, B] we 

 must distinguish a cephalo-thoracic and an abdominal division. The former often has 



5 pairs of diverticula. The first two diverticula may anastomose with each other over 

 the sternal side of the thorax and so form a ring. The lateral diverticula often bend 

 round from the side towards the middle line of the body under the thoracic ganglion, 

 first, however, giving off a blind branch to the coxal joint of each limb (e.g. in Epeira 

 and many other Araneidce}. In Atypus the thoracic portion of the mid-gut has only 

 3 pairs of diverticula, the most anterior pair in this case not forming a ring. 



In the anterior portion of the abdomen of the Araneidce, the mid-gut, which is 

 here somewhat expanded, forms a considerable number of diverticula varying in size 

 and much branched ; these are united by connective tissue to the mass which is 

 erroneously called the liver. The coloured secretions occurring in some of the cells 

 of these diverticula distinguish them from the non-coloured diverticula of the cephalo- 

 thoracic mid-gut. 



The mid-gut of the Phalangidce is a tolerably spacious sac covered laterally and 

 dorsally by numerous (30) blind tubes. These blind tubes enter the mid-gut through 



6 lateral and 1 anterior pair of apertures. 



The mid-gut of the Acarina (Fig. 369, 0) also has longer or shorter bulgings, in- 

 vaginations, or ccecal diverticula, whose number varies. There are often 2 or 3 pairs. 



The mid-gut of the Linguatulidce is a straight tube without diverticula. 



The Malpighian Vessels. In the Scorpionidce, 2 Malpighian vessels enter the 

 hind-gut. In one species (Sc. occitanus] 4 vessels are said to occur, 2 of them being 

 branched. In the Araneidce, the Malpighian vessels consist of numerous fine branched 

 and anastomosing tubes which finally unite on each side into two collecting ducts. 

 The two ducts of one side enter the rectal vesicle by a common terminal portion. 



The tubes of the Phalangidce, formerly considered to be Malpighian vessels, are 

 said by more recent observers to emerge at the mouth. In this case they can of 

 course not be regarded as Malpighian vessels. They reojiicfi^fiy^her investigation. 



