52 SUMMARY OF CURRENT RESEARCHES RELATING TO 



basal joints of the manrlibles ; these'"''are strong /-shaped capsules, 

 0'2 mm. long and 0-04 mm. broad, and widened out in saccular form in 

 their middle. Some of the tracheae which pass into the body from the 

 air-chamber pass out directly, while others are derived from a chief 

 tracheal trunk which breaks up. No trace of spiral marking could be 

 detected on the tracheae ; they are thin tubes (0-0015 mm.), and traverse 

 the body without further division ; as they often form a close plexus 

 around the organs, they may be considered to aid in keeping them in 

 their place. It is probable that the setae at the ends of the appendages 

 have some share in respiration. There is no heart, and there are no 

 blood-vessels. In transparent species of Atax it is very easy to observe 

 how the muscular activity in the movements of the legs has an influence 

 on the circulation of the blood in them. 



The pharynx is a spindle-shaped tube, the wall of which is 

 strengthened by chitin ; this forms discs which are set at right angles 

 to the long axis, and continued into the interior, so that the whole 

 internal cavity is broken up into nine divisions, each of which is filled 

 by a bundle of strong circular muscles. A very thin canal traverses the 

 axis of the whole tube ; and it is clear that it is by the alternate con- 

 traction of the circular muscles in each division that the tube is narrowed 

 and widened ; by these means the food is pumped into the oesophagus. 

 The stomach appears to be very much like that of other Hydrachnids ; 

 with regard to the rectum, however, the author is in opposition to 

 Croneberg. 



The excretory organ is placed dorsally to the central cavity of the 

 stomach, and, as it is partly covered by blind sacs, it lies in a complete 

 groove. It is formed by a simple sac, the extent of which varies in 

 different individuals. It passes into a cylindrical tube, which, like the 

 rectum, is formed of longitudinal muscles, which are attached to the 

 anal ring. The secreting cells are surrounded by a transparent homo- 

 geneous tunica propria, and have the form of spherical vesicles of 

 different sizes ; the secretion, which is always present in large quan- 

 tities, appears to consist of a number of elongated or rounded corpuscles 

 with concentric, highly refractive, bluish rings. 



The nerve-centre of Hydrodroma is like that of other Hydrachnida ; 

 the few differences that obtain are carefully noted. The eyes appeared 

 to deserve special study, and with them there were compared those of 

 Atax, Diplodontus, and Eylais. In addition to the known two pairs of 

 eyes, the author has found in Hydrodroma a fifth, unpaired, eye, which 

 is very small, and is placed in the median depression of the dorsal 

 shield. The minute structure of the eye is always on the same funda- 

 mental type, and the differences are confined to the chitinized tegu- 

 mentary part which is converted into the lens, and to the relative 

 positions of the eyes. Those of either side are always unequal in size, 

 and the larger is always more anterior and nearer the middle line. A 

 single optic nerve is given off from the brain, and this divides into two, 

 at a varying distance from its point of origin. The end of each optic 

 nerve passes into a number of club-like structures, tyhich are united 

 into a more or less conical cup, and correspond to the rod-cells. The 

 chitinous lens is greatly thickened internally, and projects into the 

 lumen of this cup. Each of the rods is invested in an extremely deli- 

 cate envelope of connective tissue, below which are dark-violet pigment- 

 corpuscles closely packed. 



