276 COMPARATIVE ANATOMY. 



3) Appendages of tlio Hind-gut. 

 § 214. 



The glands differentiated off with the hind-gut, which is itself 

 generally short, do not secrete anything, which is of importance in 

 digestion or absorption. Their secretions are rather of the excretory 

 series. And as we have chemical evidence to show that these matters 

 resemble the urinary excretions of the Vertebrata, we may regard 

 these bodies as being excretory organs, without thereby pre- 

 judicing any relations, which they may have, in some cases, to other 

 functions. 



In the Crustacea we sometimes meet with crecal organs on the 

 hind-gut, as for example in the larvae of the Copepoda, but we 

 cannot safely form any opinion as to their significance. It is pro- 

 bable that the concretions found in the walls of their entera arc of 

 an excretory character. 



Excretory glandular organs are very generally found in the 

 Tracheata ; they arise as diverticula of the enteron, and have the 

 form of long canals, which may be simple or branched, and which 

 are often arranged in several coils or loops on the enteric canal ; they 

 open into the terminal widened portion of the enteric canal, and 

 almost always behind the mid-gut. They are known as Malpi- 

 ghian vessels, or, from their function, as urinary canals. As they 

 are formed at the same time as that portion of the hind-gut, which 

 in the embryo is developed from the ectoderm, it is not improbable 

 that they primitively opened on to the surface of the body, or were 

 derived from organs which did so. In all divisions there are two 

 chief canals, as is often seen at the point where a large number of 

 canals open and unite ; this number may therefore be regarded as a 

 primitive character. 



Among the Arachnida they are, in the Scorpionea, simple canals, 

 which run between the lobes of the liver; one pair of them is 

 branched. The urinary canals of the Aranea are much branched, 

 and united into a plexus ; they unite into two common excretory 

 ducts (Fig. 137, e), by which they open into the wide hind-gut, or its 

 crecal sac. In the Opilionida they form two long highly-coiled 

 canals ; in the Acarina, where they arc sometimes provided with 

 branches, they arc of a similar form. 



An equally small number of simple urinary vessels is found in 

 the Myriapoda ; the Julida have one and the Scolopendrida two 

 pairs. They are allied, not only by their number and simplicity of 

 structure, but also by their arrangement on the alimentary canal, to 

 the corresponding organs of many Insect larvae. 



The greatest variation iu number, arrangement, and special 

 structure obtains in the urinary vessels of the Insecta. They are 

 wanting in the Collembola, among the Aptera, and in many Thysanura 



