340 



COMPARATIVE ANATOMY 



CHAP. 



The Mid-gut Diverticula of the Malaeostraea. 



Among these we shall first distinguish those which enter at 

 the anterior end of the mid-gut from those which enter at the posterior 

 end. The former are universal. They correspond ivith the mid-gut 

 diverticula of the Bntomostraca, and are generally called livers. Taking 

 into account their physiological activity, however, the name of hepato- 

 panereas is more suitable. 



In the Zeptostraca we find 4 pairs of hepatic tubes, 3 pairs of which, one upper, 

 one lateral, and one lower, are very long and run along the mid-gut, reaching far 

 into the abdomen. The short 4th pair stretches to the front of the head. The 

 tubes join on each side to form a short wide sinus, and these 

 sinuses, uniting at the two sides, enter by a common aperture 

 the posterior end of the masticatory stomach ventrally. 



In the Artlirostraca also there are 1 to 3 pairs of diverticula 

 entering the beginning of the mid-gut which is occasionally 

 widened so as partly to surround the masticatory stomach. 

 Among these there are often 2 (in the normal Ampliipoda 4) 

 tubes running backwards along the intestine, which, according 

 to their function and the structure of their epithelium, specially 

 deserve the name of a hepatopancreas. 



The arrangement of the glands of the mid-gut in the Scliizo- 

 poda, the Oumacea, and the larvse of the Decapoda is similar 

 to that in the Lep)tostraca and Arthrostraca. They are distin- 

 guished by 3 pairs of long hepatic tubes entering the most 

 anterior portion of the mid-gut. In the Stomatopoda, distri- 

 buted along the whole length of the mid-gut, there are 10 pairs 

 of branched tufts of hepatic tubes. The adult Decapoda are 

 distinguished by the possession of a paired hepatopancreas, 

 which to the right and left enters the posterior and lower end 

 of the masticatory stomach. By means of much branching the 

 liver assumes the character of a very voluminous tubular gland 

 filling a large part of the cephalo-thorax, and falling on each 

 side into 3 lobes — an anterior, a lateral, and a posterior. If 

 we examine only the extreme forms of the cells which unite to 

 form the epithelium of the Malacostracan liver, we can distin- 

 guish two sorts of cells : first, ferment cells, whose varied 

 secretions (which may be liquid or solid, coloured or colourless) 

 digest fibrine ; second, hepatic cells, whose fatty secretion con- 

 tain a colouring matter related to the gall pigment of verte- 

 brates. In consequence of these observations we cannot describe 

 the glands of the mid-gut simply as a liver, but rather as a 

 hepatopancreas. We cannot, also, carry out a sharp distinction 

 of the cells into ferment cells and hepatic cells ; many transition forms occur. 



The glandular caeca which enter the posterior end of the mid-gut are found in 

 the AmpMpoda, i. e. in the Caprellidce and the Crevcttina. They occur as one pair, 

 except in Melita, which has only one such glandular tube. Physiologically they must 

 (in the Crevettina) be considered as urinary glands. Morphologically they cannot 

 be compared with the Malpighian vessels of the Traclieata, since they do not belong 

 to the hind-gut, but to the mid-gut. 



In the posterior portion of the mid-gut of Nelalia there is at the inner side of 



a. 



Fig. 235. — Enteric 

 canal of Asellus aquat- 

 icus (after G. 0. Sars). 

 oe, (Esophagus ; km, 

 masticatory stomach ; 

 d, mid-gat ; a, anal-gut 

 (rectum) ; I, hepatic 

 tulaes (hepatopancreas). 



