CRANGON VULGARIS. 17 



this paper ('866) pp. 134-136, I have shown to be true 

 eutoderm are utilized almost wholly in the formation of the 

 voluminous liver. Between Oniscus and Crangon, as is 

 to be expected, there are minor differences. In Crangon, 

 as we have seen, there are six (three pairs of) liver lobes 

 outlined at first. In Oniscus, on the other hand, the structure 

 is more simple. In this connection it may be noted that 

 Balfour, deriving his facts from Bobretzky, says ('80, p. 

 439) that in Oniscus the alimentary tract "is mainly if not 

 wholly formed from the proctodeum and stomodeum, " 

 while on the next page he thinks that the entoderm cells, 

 besides furnishing the hepatic epithelium, "probably also 

 supply the growth material for the later growth of the ap- 

 parent proctodeum" and on this account this portion of the 

 digestive canal "does, in reality correspond to the procto- 

 deum and mesenteron together." 



Looking at Oniscus alone, as described and figured by 

 Bobretzky, it seems to me that the whole alimentary tract, 

 from the openings of the hepatic ducts to the anus is strictly 

 proctodeal in origin, while the light thrown upon the sub- 

 ject by Crangon seems to confirm this view. The yolk or 

 entoderm cells in Crangon are larger, and differ in their 

 histological characters from those of the hind gut, and I 

 have never, seen a trace of their joining themselves to that 

 part of the canal. On the other hand, they seem to remain 

 in a passive condition until a comparatively late stage when 

 they unite, not to piece out either ingrowing portion but 

 to form the hepatic epithelium. 



In Astacus the resemblances would naturally be closer 

 and so I regard them as shown in Reichenbach's ( J 86) fig- 

 ures, especially in his PI. xiv, fig. 217. His letter "Z)" 

 ("Ubergangstelle des Mitteldarms in den Hinterdarm") 

 seems to have an arbitrary position while his "MD" ("dor- 



ESSEX INST. BULLETIN, VOL. XXI. 2 



