CBAYFISHES AND THEIR RELATIVES 159 



Since the proteins, fats, carbohydrates, and protoplasm all 

 contain carbon, when these are oxidized, carbon dioxid (CO2) 

 will be formed as one of the waste substances. All these 

 waste substances will pass out of the cells into the blood, 

 which finally conveys them to the filaments of the gills. 

 Here the waste matters pass out into the water, which, as we 

 have seen, is then forced out of the front end of the gill 

 chamber. 



116. Life history. — As in the seed-producing plants, 

 crayfishes are reproduced by means of special cells known as 

 egg-cells which in crayfishes are formed in the body of the fe- 

 male in organs known as ovaries. Before they can develop, 

 however, these egg-cells, as in the seed plants, must be ferti- 

 lized by sperm-cells, produced in spermaries of the male cray- 

 fish. After extrusion the fertilized eggs are attached by a 

 sticky substance to small appendages, known as swimmerets, 

 on the ventral surf ace of the abdomen of the female (Fig. 112). 

 Here the fertilized egg-cell develops into a many-celled em- 

 bryo, and finally a tiny crayfish is hatched. At first the 

 young crayfishes are held to the swimmerets by threads; 

 later they cling by means of their pincers, and after some 

 days become independent. At intervals in both young 

 and old crayfishes, the hard outer covering of the body 

 is shed. This shedding of the skin is called molting. 

 But for this process it would be impossible for the young 

 to grow. 



While the young crayfishes are attached to the parent 

 they are of course protected by their position, and the female 

 looks after them by looking out for herself. The food for the 

 developing embryo is stored in the egg. After hatching, the 

 young must care for themselves, and after they become in- 

 dependent they receive no protection at all. There is, there- 



