286 COMPAEATIVE ANATOMY. 



lower end ; aud wliich^ in the Balamdve, are converted into a special 

 complex of glands. This organ persists in tlie Ttoracostracaj and 

 is known as the "green gland" in the Crayfish. 



A second organ of this kind is also found in the Entomostraca, 

 but is absent in the higher Crustacea. It lies in the mantle-like 

 fold of the iutegumentj where it forms a transparent looped canalj 

 which opens below the mantle (cf. Fig. 136, g). Owing to its 

 position below the shell it is known as the shell-gland. Its inner 

 end is blind. 



There are, therefore, two kinds of excretory glandular organs in 

 the Crustacea, but it is doubtful whether they are homodynamous. 

 The second organ may be homologous with the loop-like excretory 

 organ of the Vermes, and be therefore derived from a common stem- 

 form, while it has lost its metameric signification. 



These organs — the functional relations of which cannot as yet be 

 definitely adjudged, and of which the green gland alone is distinctly 

 similar to a renal excretory organ — are not found in the Tracheata. 

 In them the function of excretion is performed by organs, which 

 have been described anatomically among the appendages of the 

 hind-gut (§ 214), under the name of urinary canals, or Malpighian 

 vessels. 



Tracheae. 



§ 222. 



The coelom of the Protracheata and Tracheata is traversed by 

 an aerating system of tubes, which, so far as is yet known, is derived 

 from tegumentary organs. The characters of these organs, in 

 Peripatus, is of the greatest significance as to this point ; irregularly 

 distributed tufts of fine tubes, filled with air, are distributed on 

 the inner surface of the body-wall, and also on the oviducts and on 

 the fore^ and hind-gut. 



The arrangements in the Tracheata are different from this, for, in 

 them, the tracheae are regularly arranged and symmetrically dis- 

 tributed. They consist of an outer layer of connective tissue 

 (Fig. 149, a), the interior of which is covered by a chitinous layer 

 continuous with the external integument. Its elasticity is almost 

 altogether due to its chitinous layer, and when the trachea is more 

 elastic it is because this layer is thicker ; in this case it has the 

 form of a spiral filament, projecting into the lumen of the trachea. 

 At certain points the tracheae form saccular enlargements, and at 

 them this spiral arrangement of the thickenings disappears ; that is 

 to say, it is only deposited at certain disconnected parts. This 

 chitinous layer and its spiral ridges do not form a specific arrange- 

 ment, for the ducts of many glands have a very similar structure in 

 the Tracheata. 



The external orifices (stigmata) of the tracheae are regularly 

 arranged in pairs on either side of the body ; there is not always the 



