196 HIGH SCHOOL ZOOLOGY. 



phoral cells, (surrounded by four pigment cells), which secrete the crys- 

 talline cone, and this is prolonged inwards into a tube (formed by seven 

 cells of the innermost zone — the retinulse), of which it forms a spindle- 

 shaped core — the rhabdome. The nerve fibre to each ommatidium oc- 

 cupies the axis of the rhabdome and of the crystalline cone ; the cones, 

 therefore, constitute the sensitive elements of the eye, like the rods 

 and cones of the Vertebrates. 



Auditory sacs are present in the basal joints of the anten- 

 nulfe. Tliey contain foreign particles, which play tlie part of 

 otoliths (I, 46), and the sensitive elements are stiff hairs in 

 which nerve-fibres terminate. Both pairs of feelers obviously 

 act as tactile organs, but peculiar setae on the outer branches 

 of the antennulse have been interpreted as olfactory in function. 



When the various jaws have been removed, the mouth is 

 exposed, bounded in front and behind by unpaired chitinous 

 outgrowths, the labrum and metastoma. The chitinous cuticle 

 is continued into the spacious stomach, where it forms numei'ous 

 calcified teeth, of use in comminuting the food. Digestive 

 juices are furnished by the so-called liver, a bulky tubular 

 gland which lies above and behind the stomach, and which 

 opens into the mid-gut, the only part of the intestine destitute of 

 chitin. Behind this is the straight rectum, the lining of which 

 becomes continuous with the cuticle at the anus. 



10. In compai-ison with the Vertebrates, the Arthropods have 

 a less complete blood-vascular system, for, during part of the 

 circulation, the blood flows in intei'spaces instead of closed 

 capillary vessels. These are, however, partly represented in the 

 crayfish, and the heart, as well as the arteries and veins, is 

 well developed (Fig. 122). The blood is driven out to the 

 whole system through the arteries, and is returned by venous 

 sinuses through the gills to the pericardial sac. 



11. The female crayfish may be found in spring with eggs 

 attached to the abdominal appendages, to which the young ad- 

 here until they have attained the form of the adult. In the 



