i2 4 COMPARATIVE ANATOMY 



modified setae on the under sides of most of its joints ; these 

 are known as Leydig's organs, and are supposed to serve 

 as an olfactory apparatus, The basal joint of the first 

 antenna is triangular in section ; the upper side, corresponding 

 to the base of the triangle, is concave, and in the middle of 

 it there is a narrow slit-like aperture guarded by a fringe of 

 closely set setae. This is the aperture of the auditory organ, 

 the structure of which will be referred to later. 



The course of the gut in the crayfish is shown in fig. 28. 

 It will be observed that it has the same relations as the gut 

 of Apus, the oesophagus being almost at right angles to the 

 longitudinal axis of the body, the remainder of the gut being 

 straight, situated in the mid-dorsal line, parallel to the 

 longitudinal axis of the body, and ventral to the heart, and 

 anterior and posterior blood-vessels, which emerge from the 

 heart. The difference in detail is, however, considerable. 



The oesophagus of the crayfish leads into a capacious sac, 

 commonly called the stomach, which, like to the similarly 

 named organ in Apus, lies in the head region. But there are these 

 differences, that the " stomach " of the crayfish is lined by a 

 chitinous cuticle, whereas that of Apus is not, and the digestive 

 glands of the crayfish do not open into the so-called stomach, 

 but into a smaller sac lying behind it, which, for reasons which 

 will presently become apparent, should properly be called the 

 stomach. The large sac lined by chitinous membrane we will 

 call the proventriculus. This name has the possible incon- 

 venience that it Tias"~been already applied to another and 

 different structure, the gizzard of the earthworm ; but if the 

 fact that the two organs have no relation to one another is 

 borne in mind, the practical inconvenience will be very slight. 



The proventriculus of the crayfish, then, is lined by a 

 chitinous cuticle continuous with that of the oesophagus, and 

 the cuticle of the oesophagus is in turn a continuation of the 

 external hardened and calcified cuticle forming the exoskeleton. 

 In fact, both oesophagus and proventriculus are formed from 

 the stomodaeum, which in the crayfish is of very much greater 

 extent than it is in Apus, and the true stomach is therefore 

 pushed back, and lies in the thorax. The proventriculus is 

 divided by a constriction into a larger anterior portion, called 

 the cardiac division, and a posterior smaller portion called the 

 pyloric division. The name "cardiac" was given to the 



