188 MB. A. D. MICHAEL ON AN [Mar. 5, 



always occur; wlien they do they are not usually bilaterally 

 symmetrical, or probably in any way permanent ; they often are 

 found to a lesser degree in other parts of the sac, and evidently 

 provide for considerable extension and contraction of the lumen of 

 the organ. The folding sometimes is found in directions where it 

 would probably not result from dorso-ventral compression. 



In regard to the homologies of the alimentary canal and 

 excretory organs, it may not be immaterial to remember that 

 Wagner ' has lately found that in the embryo Ixodes the so-called 

 Malpighian vessels are formed from the endoderm quite separately 

 from the proctodeum, and only become connected with that organ 

 in the latest stage of development. The histology of the alimen- 

 tary canal does not present any features varying sufficiently from 

 what has been described by other authors to make it necessary 

 to notice them. The tunica propria is particularly clear and well 

 marked. The lumen of the ventriculus is large, its walls composed 

 of more closely-placed cells, forming a more even layer than is 

 usually found in the ventriculus of Acarina ; the cells are large 

 but not so loose nor rounded as in most species, and the large 

 groups of rounded cells projecting into the lumen and gradually 

 becoming detached and dropping off into it, correctly figured by 

 Henkin in Tromhidium, are far less abundant here. These 

 remarks apply specially to the male, in which, as far as I have 

 seen, the amount of food-material absorbed by, and contained 

 within, the cells of the ventriculus is less than in the case of the 

 female, where the cells are often greatly distended by it. 



Salivary Glands (Plate VIII. fig. 16; Plate IX. fig. 23). 



I use the expression " salivary glands " for the glands which I 

 am about to treat of because that expression is in general use for 

 them ; I am not, ho-wever, satisfied that it quite correctly expresses 

 their function. 



These glands are often largely developed in the Acarina, pro- 

 bably most so in the predatory kinds. It is already well known 

 that some species of Hydrachnidse are amply provided with them ; 

 in the present species they assume considerable importance. 

 Croneberg found three pairs of glands, each pair having bilateral 

 symmetry, two pairs being more or less kidney-shaped, while the 

 third pair are moi'e sausage-shaped. Croneberg only draws and 

 describes the portion of these later glands near to, and including, 

 the efferent end, apparently not having traced them further. In 

 his fig. 33 he draws the kidney-shaped glands as composed of 

 numerous, largish, closely-pressed secreting-cells \^ith clear nuclei, 

 and the sausage-shaped glands as composed of a single layer of 

 squarish cells surrounding a small lumen ; he shows the three 

 glands on each side of the body as communicating by small ducts 

 with a larger joint efferent-duct. Schaub also found three pairs 



1 "Beitriige ziir Pliylogenic der Aracljiiiden," Jena. Zeitschr. f. Med. u. 

 Naturw. 29 Bd. (1894)'Heft i. pp. 125-152, 



