504 The Animal Kingdom 



Tactile hairs are located on many parts of the body and appendages, 

 and chemically responsive hairs are found on the antennae, antennules, 

 and mouth parts. These sense organs are delicate structures which 

 contain sensory hairs connected to nerves. 



The endocrine system has been demonstrated in but few of the 

 invertebrates, but it has been definitely shown that the crayfish has a 

 well-developed one. The chief endocrine gland is that located at the 

 base of the eyestalk, the sinus gland. Hormones from it control the 

 molting of the animal, the expansion of pigment cells, the formation 

 of the exoskeleton, behavior patterns, and many other activities. In a 

 limited way, this gland may correspond in function to the pituitary 

 gland of vertebrates. 



The sexes are separate in the crayfish. The white testes of the 

 male are located just ventral to the pericardial sinus. From each side, 

 the coiled vas deferens passes ventrally and opens to the outside at the 

 base of the fifth walking leg. Sperm transfer is accomplished by the 

 modified first two swimmerets of the abdomen. In the female, the 

 ovaries are located in a position similar to that occupied by the testes. 

 The oviducts pass ventrally, opening near the bases of the third walking 

 legs. Ventrally the opening is surrounded by a swollen rim, the an- 

 nulus ventralis, within which is the seminal receptacle. 



The Life History of Cambarus. — The time of mating varies with 

 the different species, but in general it occurs in the fall. During copu- 

 lation, the sperm from the male are transferred by means of the modified 

 swimmerets to the seminal receptacle of the female. During this process, 

 the female lies on her back with the male holding her firmly in place by 

 means of his two chelipeds. The sperm remain for varying periods in 

 the seminal receptacles, but usually it is the following spring before 

 fertilization occurs. Shortly before laying the eggs, the female care- 

 fully cleans the swimmerets. She then lies on her back with the abdomen 

 sharply curved. The eggs are extruded, fertilized by sperm from the 

 seminal receptacle, and then attached to the swimmerets by means of 

 a sticky secretion. Following this, the female backs into a safe spot 

 with the tiny eggs hanging from the swimmerets. By constant move- 

 ments, the eggs are aerated and development proceeds for about five 

 weeks. At the end of this time, the tiny crayfish hatch, but remain 

 attached for several molts to the female's swimmerets. 



Inasmuch as the individual tgg has a great deal of yolk, the cleav- 

 age planes are superficial. 



