of the; parasitic isopoda. 
517 
of the adult consists practically of the fore- and hind-guts alone, the liver being the 
only permanent representative of the hypoblast. The mid-gut in these animals is 
therefore only a temporary structure. That this interpretation of the facts is the true 
Diagrams illustrating successive stages in tlie development of the alimentary canal. 
HG. Hind-gut. MG. Mid-gut. PG. Fore-gut. L. Liver. 
one, is rendered more probable by the fact that among certain other Crustacea the mid- 
gut is formed from the yolk-membrane. 
Fig. 4 [woodcut), copied from Van Beneden’s paper on Hessia colorata* shows 
this very clearly, and corresponds almost exactly with the woodcut, fig. 3, A. Van 
Beneden does not give a detailed account of the mode of development of these parts, 
but mentions incidentally that the mouth and oesophagus are formed by involution. 
He thus describes the figure : “ L’cesophage relie la base de la trompe buccale a 
1’elargissement considerable du tube digestif (yolk-membrane) qui renferme les restes 
Fig. 4. 
Hessia colorata (Van Beneden, Bull. Acad. Roy. Belg., t. xxix., 1870, pi. xxix., fig. 15). 
du vitellus ; il s’est considerablement allonge, et Ton distingue facilement ses parois. 
On peut suivre le tube digestif dans toute sa longueur : fortement renfle en avant, 
il se retrecit progressivement en arrier (hind-gut) pour s’ouvrir a l’extremite 
posterieure de l’abdomen entre les deux appendices caudaux. Il renferme le reste 
de la masse deutoplasmatique qui a singulierement diminue dans le cours du 
developpement embryonnaire, ayant ete employee it former successivement les 
differents organes de l’embryon.” 
* Bull. Acad. Roy. Belg., t. xxix., 1870, pi. xxix., tig. 15. 
