84 



ARACHNIDA. 



of the intestine and the position of these vessels in the adult. We cannot, 

 however, regard this point as fully established, although our own investigations 

 made the entodermal character of the tubes appear highly probable.* Further, 

 it lias been definitely stated that the Malpighian vessels of the Scorpiones arise 

 from the entoderm (p. 20), and we regard the statements made as to their ecto- 

 dermal origin to be far less trustworthy. 



a. 



Fig. 43. — A, longitudinal sections. B and C, transverse sections through young embryos of 

 Theridium maculatvm (after Morin). b, blastoderm; bl, blood-corpuscles; d, yolk; dz, 

 yolk-cells; ec, ectoderm; kl, cephalic lobe; mes, mesoderm; si, caudal lobe; I-VI, first six. 

 segments. 



The derivation of the so-called Malpighian vessels from mesoderm-strands,. 

 and that of the rectal vesicle from an unpaired coelomic sac belonging to the 

 caudal region (Kishixouye, No. 62) are, from what we know of the manner of 

 origin of these vessels, so improbable that we need not enter further upon it.t 



* In sections of young Araneids (Tegcnaria domestica), which were kindly 

 placed at our disposal by Dr. A. Brauer, the formation of the intestine followed 

 the course described above for Agalena; the posterior trumpet-shaped rudiments 

 of the enteron had already opened ventrally into the proctodaeum. It appeared 

 exactly as if the Malpighian tubes arose from an entodermal part, but this point 

 can only be decided with certainty at a somewhat earlier stage when the enteron 

 ami the proctodaeum are not yet united. From the length of the tubes at the 

 stages under consideration, we may conclude that their rudiment is to be found 

 earlier. 



t [See foot-note, p. 81. — Ed.] 



