INTERNAL ANATOMY OF BDELLA. 519 



thej are in si(n, they actually toucli along their whole length— indeed, for the greater part 

 of their course, they are surrounded hy a single enveloping tunic common to both. These 

 two vessels are striated, the air-chamher very finely and regularly, the tracheal trunk 

 much more coarsely and irregularly. The tracheal trunk enlarges somewhat near its 

 anterior end, and when it has arrived at the termination of the air-chamber it tm*ns 

 suddenly upward behind the mandible, forming a somewhat curved air-passage, which is 

 almost divided uj) into sections by partial constrictions and irregular broken semi- 

 partitions or laminae jn-ojecting into the lumen. At the upper end of this jiortion of the 

 tracheal trunk is the stigma (S), of which there is only the one pair in the body, 

 communicating with the small external air-chamber between the mandibles. There are 

 not any other tracheal trunks in the body, but from the ventral side of the posterior 

 thu'd of tlie air-chamber, and from the actual posterior end and a small portion of the 

 dorsal side of the chamber, there arise a multitude of extremely fine, unbranched, and 

 unstriated tracheae (^r), which are distributed to all parts of the body ; only the 

 commencement of them is shown in the drawing. Some of these tracheae turn forward 

 and serve the tropin and anterior organs, some run backward and turn forward afterwards ; 

 but far the greater number of them run backward toward the posterior end of the 

 creature. A thick fasciculus of these small tracheae tiu'ns across the body immediately 

 behind the supra-oesophageal ganglion of the brain, thus lying upon the sub-oesophageal 

 ganglion and upon the oesophagus itself. The fasciculi from the two sides of the body 

 meet and cross behind the supra-oesophageal ganglion, the posterior side of which is almost 

 hidden by them until they are removed. In Bdella capilluta, Kramer, which I examined, 

 the arrangement is similar, except that the number of fine tracheae is very much less 

 than in B. Basteri, and that the tracheal trunk does not enlarge until it turns ujjward ; 

 but then it enlarges comparatively more than in B. Basteri. 



The Dermaiv Glands. (Fig. 10.) 



Instead of assuming the importance which the similar organs do in the Hydi'achnidae, 

 the dermal glands of Bdella are apparently few in number (I have only traced one pair) 

 and are extremely small and difficult to find ; when found, however, they bear a 

 resemblance to those of the Water-Mites, but on a very much smaller scale, and are 

 simpler. In B. Basteri they consist of minute sacs, averaging about 25 ^i in extreme 

 length, placed immediately beneath the hypoderm, each having one small duct placed 

 excentrically, which passes to the outer surface of the cuticle, where its mouth forms a 

 very small pore which is not protected hy any sj)ecial hair or spine, l)ut which is doubtless 

 sheltered by the sharp wavy ridges with which the whole cuticle of Bdella is covered. 

 The contents of the sac are a few masses of protoplasm which stain freely, and which are 

 joined by irregular protojilasmic strands. It is possible that they may not be homologous 

 with the dermal srlands of the Hvdrachnidse. 



a' 



The Endosternite. 



I have not figured this organ because it is of the type well known in the Acariua, 

 e. g. the Gamasidae and other forms ; but I mention its existence, particularly as it is 



70* 



