STRUCTURE OF THE CRAYFISH. 241 



copulatory modification of the first abdominal appendages. 

 In both sexes the reproductive organs are three-lobed, and 

 communicate with the exterior by paired ducts. 



The testes consist of two anterior lobes lying beneath and 

 in front of the heart, and of a median lobe extending back- 

 wards. Each lobe consists of many tubules within which 

 the spermatozoa develop. From the junction of each of the 

 anterior lobes with the median lobe, a genital duct or vas 

 deferens is given off. This has a long coiled course, and 

 ends in a short muscular portion opening on the last thoracic 

 limb. The spermatozoa are at first disc-like cells, but they 

 give off on all sides long pointed processes like those of a 

 Heliozoon. The seminal fluid is milky in appearance, and 

 becomes thicker in its passage through the genital ducts. 



The ovaries are like the testes, but more compact. The 

 eggs are liberated into the cavity of the organ, and pass out 

 by short thick oviducts opening on the third last pair of 

 walking legs. As they are laid they seem to be coated with 

 the secretion of the cement glands of the abdomen, and the 

 mother keeps her tail bent till the eggs are glued to the 

 small swimmerets. 



Before this, however, sexual union has occurred. The 

 male seizes the female with his great claws, throws her on 

 her back, and deposits the seminal fluid on the ventral 

 surface of the abdomen. The fluid flows down the canal 

 formed by the first abdominal appendages, and these seem 

 to be kept clear by the movements of the next pair, which 

 are also modified. On the abdomen of the female the 

 agglutinated spermatozoa doubtless remain until the eggs 

 are laid, when fertilisation in the strict sense is achieved. 



Development. — The ovum has a central core of yolk, and 

 is surrounded by a firm membrane. After fertilisation, 

 peripheral segmentation begins, the central portion remain- 

 ing for a while unchanged. A blastoderm or area of seg- 

 mented cells extends all over the yolk. 



The parts of the embryo begin to be mapped out on one 

 portion of the ball of cells, known as the " ventral plate," 

 although in reality it lies uppermost. The anterior region 

 of this ventral plate shows the outlines of the head-lobes, 

 behind these are hints of the appendages. 



On a special area known as the " endoderm-plate,'' a slight 

 Q 



