254 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Crustacea. The simplest is that which obtains in 

 many Copepocla, where there is merely a long 

 smooth tube, of the same calibre throughout ; in some 

 Phyllopods the tube is enlarged at certain points, 

 and more especially at its blind end ; while the third 

 and most complex stage is that which obtains in the 

 crayfish, where the tube is widened at various points, 

 has the constituent cells differing in structure and 

 function, and is folded on itself. We may suppose 

 that the lower terminal portion is glandular and ex- 

 cretory, and that the wide thin-walled sac acts as a 

 reservoir. 



The organ of Bojanus in the lamellibranch 

 Mollusca offers many very striking points of re- 

 semblance to the green gland, but it differs most essen- 

 tially in retaining the primitive character of having an 

 opening into the body cavity. On the floor of the 

 space which surrounds the heart (pericardium) we 

 find, on either side of the ventricle, a small orifice 

 which leads into an elongated chamber, with thick 

 dark-coloured walls, and narrower at its hinder than 

 at its front end ; the walls give rise to spongy out- 

 growths, which project into the cavity, and which 

 contain blood spaces, and are invested by the secreting 

 epithelial cells ; at its hinder and narrower end this 

 thick- walled portion opens into a cavity which lies 

 above it, and which has thin walls ; this, which 

 opens on either side into a cloaca, or directly to the 

 exterior, is no doubt the portion of the organ which 

 has the function of a reservoir. Here, then, we have 

 again an arrangement which may be explained as 

 that of a tube, folded on itself, and having part 

 differentiated into a glandular secreting region and 

 part into a collecting region or reservoir. The gland 

 is said to secrete uric acid. 



In the fresh-water mussel the products of the gene- 

 rative glands pass to the exterior qiiite independently 



