240 ANIMAL BIOLOGY. [Part II. 



the surrounding water by a filmy chitinous membrane. Respira- 

 tion is effected by the absorption of oxygen dissolved in the 

 water, and the giving up of carbonic acid. 



External Apertures. 1. On the concave upper face of the 

 basal joint of the antennule is an elongated aperture, guarded 

 by setae, which opens into the auditory sac. 



2. On the basal joint of the antenna is a conical process, near 

 the apex of which is a small orifice, the opening of the green 

 gland or renal organ. 



3. Between the mandibles is a large longitudinal slit, the 

 mouth. 



4. On the basal joints of the second pair of legs are the 

 generative apertures of the female. 



5. On the basal joints of the fourth pan\ of legs are the 

 generative apertures of the male. 



6. Beneath the telson is the anal aperture. 



General Internal Anatomy. When the tergal portion of the 

 cephalo-thorax and abdomen of the crayfish is removed by 

 snipping it away with scissors, and the underlying skin is care- 

 fully stripped off, the following organs are visible without 

 further dissection (Fig. 75). Anteriorly in the middle line is 

 the rounded sac-like stomach (st.). A whitish bar crosses it 

 transversely, the cardiac ossicle (c. 0s.), and to its anterior edge 

 are inserted two anterior gastric muscles, which are attached to 

 small (procephalic) processes at the base of the rostrum. Smaller 

 muscles (cardio-pyloric) pass back from its hinder edge towards 

 a second shorter transverse bar, and wing-like bars attached to 

 it. This bar is the pyloric ossicle. From this posterior bar 

 and its wings pass back two posterior gastric muscles (pg.) t 

 which are inserted in the carapace. Between them is a levator 

 muscle (I. m.). On either side of the stomach are the cut ends of 

 the powerful muscles (m. m.) by which the mandibles are closed. 

 At the sides of the anterior end of the cephalo-thorax, situated 

 low down, are two greenish oval masses, the green glands 

 (gr. gl.)^ surmounted on the inner side by a delicate sac (5.). 



