INTESTINAL (,'ANAL. 173 



and can thus be described as epipodial gills (p. 195), as homologous 

 structures throughout the whole of the class Crustacea. We may 

 also, perhaps, derive them from the branchial tubes of the Annelida. 

 On the other hand, it must be pointed out that branchial outgrowths 

 develop at other points also, e.g., on the exopodite of the pleopoda 

 (Squilla), or on the endopodite of the same limbs (Siriella), as 

 dorsal appendages in certain Ostracoda (Asterope), as mantle-folds 

 in the Balanidae, etc. These naturally cannot be homologised with 

 the epipodial gills. The epipodial branchial sacs present in a single 

 row in the Phyllopoda are replaced in the Decapoda by three rows 

 of branched tubes, which, according to their exact points of attach- 

 ment, are distinguished by Huxley as podobranchiae, arthrobranchiae, 

 and pleurobranchiae. Instead of these, we find in the Euphausidae 

 and Lophogastridae only a single row of dendriform branched tubes, 

 so that Claus raises the question whether the three rows of gills 

 of the Decapoda may not be derived from the principal branches of 

 the Schizopodan gill which have shifted apart. 



F. Intestinal Canal. 



The intestinal canal arises here, as in most groups of animals, 

 from three separate rudiments, the fore-gut and hind-gut arising as 

 ectodermal invaginations (the stomodaeum and the proctodaeum), 

 while the mid-gut is formed from the cells of the entoderm. Whereas 

 the two former approach their adult form by a series of comparatively 

 simple changes, the development of the latter is brought about by far 

 more radical ontogenetic processes, on account of disturbances due to 

 the presence of the food-yolk, these processes also varying in the 

 different orders of Crustacea. 



There is some variation among the sub-groups of the Crustacea with regard to 

 the time of the appearance of the fore- and hind-guts. In the Entomostraca, 

 the rudiment of the fore-gut, as a rule, appears first. This is also the case in 

 Asellus, Gammarus, and Mysis, while, in Oniscus, the hind-gut develops first. 

 In the Decapoda, the proctodaeal invagination usually appears first, a fact 

 connected with the early development of the abdomen. 



The position of this invagination with relation to the closed blastopore is of 

 importance. In Moina, according to Grobben, the point at which the blastopore 

 closes corresponds to the oral aperture, while in the Decapoda, it always lies close 

 to the future anal orifice. According to Reichenbach, in Astacus, it lies some- 

 what behind the point at which the anal aperture is forming, as is also the case 

 in Atyephyra (Ischikawa). The opposite, according to Lebedinsky, holds 

 true for Eriphia. Here the proctodaeal invagination develops behind the 

 blastopore. Kingsley, on the contrary, believes that, in Crangon, the procto- 

 daeal invagination corresponds exactly to the point at which the blastopore 



