362 



ANIMAL LIFE-HISTORIES 



ways by which the early stages of life are protected, extreme 

 fecundity is the most important safeguard against the extinction 

 of species. With Arthropods we commence the study of Higher 

 Invertebrates, which possess an extremely complex structure, and 

 afterwards pass on to Vertebrates, the organization of which is 

 even more elaborate. It naturally follows that in all these higher 

 groups the development is more or less protracted, and there 

 is therefore a greater necessity for elaborate arrangements by 

 which eggs and young are protected. Such arrangements have 

 reached a high degree of perfection in many cases, though marked 

 fecundity is also characteristic of some higher animals. If the 

 eggs or young are well protected, so that they have a good 

 chance of becoming adult, they are produced in comparatively 

 small numbers. But where the survival of the species depends 

 chiefly on fecundity, an enormous number of eggs or young may 

 be brought into existence. 



LIFE-HISTORIES, &c., OF CRUSTACEANS (Crustacea) 



Lower Crustaceans (Entomostraca). — The eggs and some- 

 times the newly-hatched young are here for a time protected by a 

 variety of arrangements, which give them a better start in life 



than would otherwise be the case. Some 

 of the Leaf-Footed Crustaceans [P/iyilo- 

 podd) illustrate this point very well. In 

 Apus, for example, one pair of limbs are 

 modified into brood-pouches (fig. 884) 

 comparable in appearance to a couple of 

 watch-glasses applied together. These 

 shelter the eggs from the time of laying 

 until the larvae are hatched out. The 

 little Water- Flea {Daphnia pulex, fig. 

 885), so common in fresh water, uses the 

 space between the upper side of the body and the enclosing shield 

 as a brood-pouch. In this case the development is direct, there 

 being no free-swimming larva, and the comparatively large eggs 

 contain a good deal of yolk, which serves to nourish the embryos. 

 The brood-pouch also contains a nutritive fluid employed for the 

 same purpose. A lucky chance sometimes enables us to see the 

 young Water- Fleas hatched out under the microscope, that is if 



Fig 



-Brood-pouch of Apus, enlarged 



