THE INTESTINAL CANAL AND ITS APPENDAGES. 83 



splanchnic layer of the mesoderm. At the points Avhere the isolated 

 complexes remained connected with the principal mass of yolk, the 

 efferent ducts of the liver arise. The lobed structure of the latter 

 results from further bulgings of its wall. There appears to be some 

 confusion regarding the origin of the caeca of the thoracic gut, they 

 are probably entodermic (cf. p. 91). According to Loot, these caeca 

 of the stomach extend (in Agalena) into the bases of the limbs, a 

 feature which would strikingly recall the Pantopoda. 



The final shaping of the intestine does not take place until a late 

 stage. When the Araneid hatches, the two principal rudiments of 

 the enteron have not yet united ; the chief part of the yolk is 

 still present, and the young animal cannot therefore feed indepen- 

 dently during the first part of its free life. 



The formation of the intestine is treated in a somewhat similar manner l>y 

 Locy and by Morix, whose accounts are also in general agreement with those of 

 Balfour and Schimkewitsch. The few points on which these authors differ 

 are not important. We must, however, devote some attention to an important 

 appendage of the intestine, the so-called Malpighian vessels, as to the origin of 

 which authors differ. 



Two long tubular appendages of the intestine, opening into it 

 almost at the point where the metenteron passes into the proctodaeum, 

 ■are regarded as Malpighian vessels. In describing the formation of 

 the proctodaeum, it was mentioned that it widens to form the rectal 

 vesicle, also known as the cloaca. In Agalena, in which this point 

 has been best investigated, the rectal vesicle lies dorsally, for the blind 

 ■end of the enteron becomes connected with the ventral wall of the 

 proctodaeum somewhat far back, so that the greater part of the vesicle 

 lies in front of the junction of these two parts of the intestine. In 

 Theridium and Pholcus, however, the enteron opens into the anteii< t 

 •end of the vesicle, if we may judge from Morin's figures (Fig. -41 B). 

 It appears to be very difficult, after the union of the enteron and the 

 proctodaeum, to decide to which of these the different parts belong. 

 This accounts for the different opinions of various authors as to the 

 point at which the Malpighian vessels originate. While Balfour. 

 Schimkewitsch, and Morin state that they arise from the procto- 

 daeum, Locy and Loman consider them to be of entodermal origin. 

 The statements of these last two authors are more definite than those 

 of the other writers named above, the former having paid less 

 special attention to this point. 



Locy states with considerable certainty that the Malpighian vessels arise from 

 the tubular section of the posterior rudiment of the enteron, and Lomax founds 

 his decided opinion of their entodermal nature on the histological constitution 



