THE INTESTINAL CANAL AND ITS APPENDAGES. 81 



millae, the four large ones being derived from the exopodites of the 

 fourth and fifth pairs of abdominal appendages, while the small inter- 

 mediate mammillae are derived from the endopodites of the fifth pair, 

 the endopodites of the anterior pair being either altogether wanting 

 or else concerned in the formation of the cribellum or collulus 

 (Jaworowski, Thorell, Pocock). Reduction appears to have gone 

 furthest in the Tetrapneumones, where it is stated that only four 

 spinning mammillae are present, possibly representing the exopodites 

 and endopodites of a single pair of abdominal appendages. In all 

 cases, the exopodites exhibit segmentation which is wanting in the 

 endopodites. There can be no doubt that both structurally and 

 developmentally the mammillae are true limbs.] The spinning glands 

 themselves must, as already stated, be regarded as crural glands, and 

 the same significance must be ascribed to the poison glands, which 

 arise as ectodermal thickenings at the tips of the chelicerae and grow 

 inward from that point (Schimkewitsch). 



F. The Intestinal Canal and its Appendages. 



The stomodaeum has already come under our observation as an 

 invagination occurring early between the cephalic lobes, near their 

 posterior margin (Fig. 28 B, p. 52). This invagination lengthens 

 posteriorly (Fig. 41 A and B), and becomes differentiated into the 

 pharynx, the oesophagus, and the sucking stomach. Strong muscle - 

 strands, inserted upon the first and last of these, run to the body- 

 wall (Fig. 41 A and B, mu). These are : a strand running from the 

 pharynx to the dorsal part of the cephalo-thorax, another running 

 from the sucking stomach in the same direction, and two lateral 

 muscles extending from the latter to the edges of the sternal plates. 



The proctodaeum, like the fore-gut, consists of several sections. 

 It appears at a late stage, when the flexure-reversal is already far 

 advanced, from an invagination of the last segment (Fig. 41 A, a), 

 and grows forward ; widening anteriorly, it gives rise to the rectal 

 vesicle* (Fig. 41 B, rb), while a short posterior portion, the true 

 rectum, remains tubular. 



* [The rectal vesicle, or stercoral pocket, usually regarded as a derivative of 

 the proctodaeum, was stated by Kishinouye (No. 62) to arise from the meso- 

 derm. Recently he has re-investigated this point, which, as he states, is 

 inexplicable, and has been forced to the same conclusion. He finds in Lycosa 

 and AgaJcna a maximum of fifteen pairs of mesoderm-segments and cavities : 

 and just after the degeneration and fusion of the three posterior abdominal 

 segments a new unpaired cavity appears in the mesoderm of the caudal lobe, 

 and from this the stercoral pocket arises (App. to Lit. on Araneae, No. VI.). 

 Laurie finds in Phrynus (App. to Lit. on Pedijialpi, No. I.) that the stercoral 

 pocket is derived from the enteron ; he thinks that Kishinouye was misled by 

 the involved condition of the entoderm in the Araneae. He agrees with this 

 author that this organ does not arise from the proctodaeum. — Ed.] 



G 



