DIGESTIVE CANAL OE ARTHEOPODA. 



275 



Fig. 143. Enteric canal and liver of Crustacea. 



J.OfOniscus. BOf a larval Palinurus (Phyl- 



losoma). v Masticatory stomach, i' Chyle-stomach. 



a Anus. /* Hepatic tubes. 



grouped in tufts. Inasmuch as in the larvas of the Decapodfl they 

 are mere diverticula of the wall o£ the enteron, it is clear that 

 they are only more de- 

 veloped stages of those A - $ 

 simpler tubes, which are 

 found in many Eutomos- 

 traca. 



A second form of this 

 hepatic organ is distin- 

 guished from the former 

 by the larger number of 

 its separate glands, and 

 by the more backward 

 position of its opening 

 into the mid-gut. Indi- 

 cations of this are even 

 found in the Copepoda 

 in the form of several 

 succeeding diverticula of 

 the mid-gut. They are 

 more developed in some 

 Isopoda (Bopyrus), where 



they beset the whole of the mid-gut in the form of paired and 

 branched tufts of glands. In the Stomapoda also we find a larger 

 number (10 pairs) of lobate tufts of glands placed all along the 

 mid-o-ut. 



The two forms cannot be directly derived from one another, for 

 those parts which carry glands in the former groups do not 

 do so in the others. It is possible that both kinds of organs were 

 united in a common stem-form. If so, we can imagine the whole 

 mid-gut beset with caeca, from which two sets of glands were de- 

 veloped ; in one the most anterior pair were developed, and in the 

 other suppressed, whilst the hinder pair was more or less largely 

 developed. These hinder glands distinguish the mid-gut of the 

 Poecilopoda, where they have the form of two pairs of branched 

 tufts. 



Among the Tracheata we find similar differentiations of the wall 

 of the enteron in the Arachnida ; in which division we must regard 

 them as acquired organs only. The anterior pair are not, however, 

 always developed into glandular organs, but persist as more or less 

 wide pouches or tubes ; these have been already fully described as 

 stomachal casca (§ 209). In the Opilionida they are exclusively 

 glandular in character. In the Scorpionea and Aranea separate tufts 

 of glands open into the hinder portion of the mid-gut. The Aranea 

 have two or three (Fig. 137, h), the Scorpionea five pairs. 



These appendages are not found in the Myriapoda or Insecta, 

 and the absence of diverticula from this portion of the enteron during 

 development shows that any diverticula, which appear in it, have 

 merely a secondary significance. 



t 2 



