INTERNAL ANATOMY OF 13DELLA. 497 



dissected out, is naturally deep red, which makes it conspicuous. TIic form of the organ 

 varies considerably in difTerent species, and even varies in different specimens of the 

 same species ; but such variations in the one species only involve differences in the 

 proportion of length to breadth, and of the comparative lengths of the anterior and 

 posterior points, and such matters ; the general plan of tlie form is always similar in 

 the same species, so far as I have seen. 



The shajje, being rather complicated, will be much better understood from the 

 drawings than from descriptions. In Bdella Basteri the ventral surface of tbe organ, 

 wiiich is pressed against the dorsal surface of the rcceptaculum cibi, is almost flat, but 

 cut away from the ventral surface at the anterior end, i. e. the dorsal part of the 

 anterior end overhangs ; the posterior end bends slightly downward ; both anterior and 

 posterior ends are broad pointed projections, Avith convex sides; behind the anterior 

 pi'ojection the gland widens suddenly, and forms a kind of shelf or platform near, but 

 not at, the ventral surface of the gland ; upon this platform, commencing a little way 

 behind its anterior edge, are two large rolls of substance arranged longitudinally, and 

 with their inner sides so closely pressed together that they often practically fuse in the 

 adults ; but in the nymphs they are distinct, and give the gland the appearance of 

 having had a double origin, or at all events that this part of it has been a horseshoe- 

 sliaped structure, which has become pressed together; this horseshoe shape is very 

 apparent in horizontal sections of the gland, even in the adults. These rolls continue for 

 about half the length of the organ ; then rolls and platform, and the rest of the organ, 

 gradually merge and lose their identity, all contributing to form the posterior portion. 

 The duct from this gland is siiort and straight, but is ratber large in diameter and 

 somewhat wrinkled longitudinally, and is evidently capable of expansion ; it springs 

 from the median line of the gland between the anterior ends of the two rolls spoken of 

 above ; in sections it may be traced some way into the substance of the gland. The 

 anterior end of the duct is enlarged, and forms an ovate membranous chamber (fig. 11), 

 which is generally more or less filled with the secretion of the gland. This chamber 

 commences behind the mandibles, but reaches a little between their proximal ends ; there 

 this gland-serving chamber discharges into a space continuous with the air-chamber from 

 which the principal tracheal trunks spring. The secretion found in the glandular 

 chamber is a yellow oily liquid, and in specimens which have been in alcohol a yellow 

 fibrous or crystalline ball may frequently be found in the chamber. 



The Brain and Nervous System. 

 (Figs. 13, 11, 15, 27, 31.) 

 The so-called brain — i. e., the great central nervous mass — in the Arachidna doubtless 

 represents an oesophageal collar, consisting of a sujira-oesophageal ganglion and a sub- 

 ousophageal ganglion joined together by commissures. In the Acarina, however, the 

 concentration of the ganglia is so extreme, and the commissures are so short and broad, 

 that the distinction of parts is usually almost or entirely lost. Thus, for instance, in the 

 llydrachnidse the brain has usually become one elliptical or almost globular organ, 

 pierced by the passage for the oesophagus, but in which it is very hard to distinguish 



