364 APPLIED BIOLOGY 



first five segments of the abdomen are similar, except that the first 

 pair is very small; while those on the first and second abdominal 

 segments of a male are modified into twisted and pointed appendages. 



The ovaries of a female crayfish lie beneath the pericardial cavity, 

 and the two oviducts open on the basal segments of the penultimate 

 walking legs (third pair). The spermaries of a male lie in the same 

 position and the sperm-ducts open on the basal segments of the last 

 (fourth) walking legs. 



At the egg-laying time, eggs issue from the oviducts and become 

 glued to the appendages of the abdomen, which is kept curved so 

 that the tail-fin partially covers the eggs. The eggs are fertilized 

 by sperm-cells which before egg-laying were deposited on the ventral 

 surface of the female near the openings of the oviducts. 



The young crayfishes when hatched cling to the appendages of 

 the mother until they are able to care for themselves. 



Muscular System. This consists of (a) muscles of the appendages, 

 and (6) body muscles. The greater part of the abdomen is composed 

 of muscles which cause its movements. Examine an abdomen cut 

 transversely and notice the position of the intestine. On right 

 and left sides there is a small dorsal mass of muscle, and a larger 

 ventral mass. Examine these in the crayfish used for study of the 

 digestive system and notice how they extend into and are attached 

 to the skeleton of the thorax. The large ventral muscles bend the 

 abdomen quickly when the tail-fin is used in swimming backwards ; 

 the dorsal muscles straighten it. 



As an example of how some muscles of the appendages are at- 

 tached inside the skeleton, move a mandible (jaw) of the crayfish 

 with carapace removed and note the movement of a peculiar conical 

 muscle which extends almost vertically. It was attached to the 

 carapace before dissection. Other appendage muscles can be seen 

 after the digestive and reproductive organs and body-muscles are 

 removed. 



Nervous Organs. Remove the organs already studied, taking 

 care not to destroy the nerve-cord, which lies on the ventral surface 

 of the abdomen between the great body-muscles already mentioned. 

 Note in each segment of the abdomen a thickened portion (ganglion) 

 of the nerve-cord. These ganglia contain masses of nerve-cells, 

 and from them small nerves extend laterally to various organs. In 

 some places the nerve-cord is seen to be double. 



On each side of the esophagus is a small nerve-cord which extends 

 anteriorly to a cerebral ganglion (popularly called "brain"), which 

 lies in the median line below the beak. Just back of the esopha- 



