174 CRUSTACEA. 



closed, this being also maintained by Bobretzky for Astacus and agreeing 

 with P. Mayer's account of Eupagurus. Hoek found that, in the free-living 

 Copeiwda, tlie point at which the blastopore had closed corresponded to the 

 position of the future anal aperture, and Nassonow made the same observation 

 in Balanus, so that, as a rule in the Crustacea, the position of the blastopore 

 may be assumed to be in the neighbourhood of the anal aperture (p. 141). 



The time at which the three rudiments unite to form a single canal varies 

 according to the stage of development at which the larva leaves the egg. In 

 the free-living Cojjepoda (Cetochilus), the intestinal canal is completed at an 

 early stage, while in the Decapoda, communication is established between the 

 fore- and hind-guts and the mid-gut usually at a later stage. The hind-gut 

 here appears to show its adult structure sooner than the fore-gut. 



We have as yet very little accurate information as to the manner 

 in which the mid-gut develops in the Entomostraca. In Moina, the 

 entoderm-cells first form a solid strand, a cross section of which 

 reveals a radial arrangement of the cells, hut no lumen (Grobbex). 

 In Cetochilus, on the contrary, the entoderm-sac which is formed by 

 invagination seems to be transformed direct into the mid-gut. In 

 many other Entomostraca, the mid-gut rudiment can be recognised 

 as a central mass of cells filled with food-yolk {Balaniis, Lang, 

 !N"assonow). In later stages, the nuclei of the entoderm-cells, with 

 the protoplasm which surrounds them, migrate to the surface, and as 

 the food-yolk is gradually assimilated, the cavity of the mid-gut 

 appears within. This is also the case in Palaemrm (p. 132). In the 

 parasitic Copepoda also, the mid-gut, according to Van Beneden 

 (No. 17), appears to develop in this way. The mid-gut, filled with 

 food-yolk, is connected at its anterior end with the stomodaeal 

 invagination, and at its posterior end with the proctodaeal invHgi- 

 nation. 



The development of the mid-gut is best known in the Decapoda. 

 In Astacus, Avhere the cells of the entoderm-vesicle absorb the 

 whole of the food-yolk, without thereby disturbing tlie conformation 

 of the vesicle, the epithelium of the mid-gut arises by the shifting 

 of the nuclei to the surface of the yolk-liearing entoderm-pyraraids, 

 a separation of the cells from the food-yolk there taking place ; as 

 the entoderm-cells increase in number they become arranged into an 

 epithelium which now covers the surface of the disappearing yolk 

 (p. 130). At the same time, the whole rudiment of the mid-gut, by 

 constriction from without, assumes a lobate form. Paired anterior 

 lobes form wliich become connected with the median rudiment of 

 the mid-gut, at whose posterior dorsal portion can be recognised 

 another swelling, the rudiment of the dorsal caecum of the mid-gut. 

 The development of the mid-gut epithelium just described first takes 



