THE INTESTINAL CANAL. 21 



far removed from the proctodaeum. This is not the case in the 



Araneae, in which the so-called Malpighian vessels arise close to 



the point where the enteron and the proctodaeum unite. In the 



Scorpiones also the large section of the intestine lying behind the 



point where the Malpighian tubes enter it has been regarded as 



the proctodaeum, i.e., looked upon as of ectodermal origin. If 



Laurie's observations should prove correct, this section, or at any 



rate the greater part of it, must be considered as entodermal, and 



the proctodaeum proper would then also be very short in the adult : 



a decided shifting of the Malpighian vessels must then have taken 



place. 



Although Kowalevsky and Schulgin do not mention the origin of the 

 Malpighian tubes, their statements as to the origin of the enteron and the 

 proctodaeum agree with Laurie's view. According to them, the tubular 

 posterior part of the enteron grows through the whole of the post-abdomen to 

 the penultimate segment, where it joins a short proctodaeum. But for this 

 statement, we might be inclined to suppose that the portion of the intestine 

 lying in the post-abdomen was of ectodermal origin, and to assume that the 

 proctodaeum ran very far forward, especially as the proctodaeum in the Araneae 

 is very long. Such an assumption, however, is incompatible with the descrip- 

 tions of Laurie and of Kowalevsky and Schulgin. We must therefore 

 regard the so-called Malpighian tubes of the Scorpiones as entodermal, although 

 we must point out the desirability of further research in connection with this 

 important point. The Malpighian vessels of the Myriopoda and the Insects 

 arise undoubtedly from the ectoderm, i.e., they are appendages of the procto- 

 daeum. In a few Crustacea, on the other hand (e.g., the Amphipoda), tubular 

 appendages are found at the posterior part of the enteron, which are probably 

 excretory, and resemble the Malpighian vessels in structure. 



The stomodaeum arises as an invagination between the cephalic 

 lobes (Figs. 6 and 13 B). The proctodaeal invagination which, 

 according to Laurie, appears at a very late stage, seems to be shifted 

 towards the penultimate segment; this corresponds to the position of 

 the anus in the adult. The two ectodermal organs, the stomodaeum 

 and the proctodaeum, only unite with the enteron at a later period. 

 Indeed, the intestine develops so late that, when the embryo is ready 

 for birth, its cells have not yet attained the regular epithelial arrange- 

 ment of the anterior part of the enteron, but some of them still 

 extend in between the masses of yolk. The cells are not distinctly 

 marked off internally, and the lumen is not yet formed. This 

 incomplete development of the intestine, and the presence of a 

 quantity of yolk within it, render it highly probable that the young 

 Scorpion does not begin to feed for some time after birth. The 

 mother is known to take care of the brood also after birth, carrying 

 the young about on her back for some time. 



