104 INVERTEBRATE ZOOLOGY. 



periods and should be studied in connection with the life history and develop- 

 ment. 



75. Develo2)meut . This should be studied by arranging a small aquai'ium 

 in a Stender dish or tumbler. Place in the bottom a layer of pond mud 

 and introduce a bit of some green alga. Cyclops will multiply in this readily 

 and very rapidly, and all stages may be found. For special study, individ- 

 uals, pairs, etc., must be isolated and kept in a smaller dish. Investigate the 

 following points : — 



(a) Males. These are much smaller than the females and of rather rare 



occurrence, only appearing at certain times. They are recognized by 

 the peculiar modification of the first antennae. Testes and vas def- 

 erens are recognizable only at time of pairing. The reproductive 

 openings are upon the first abdominal somite. The spermatozoa 

 form masses known as spermatojiho)'es, a pair of which appear dur- 

 ing pairing, as little oval refractive bodies depending from the geni- 

 tal somite. 



(b) Females. Look over several females iintil you find one in which the 



ovaries appear as dark branching masses. These lie symmetrically 

 on either side of the alimentary canal, connected by one or two com- 

 missures. After fertilization, the eggs pass out from these through 

 openings in the first abdominal somite, and form a pair of external 

 egg-masses, the ovisacs, attached by stalks to the parent. 



(c) Pairing. This may be observed only through chance, by continued 



watching during the period in which males are abundant. The 

 male clasps the first antennae about the fifth pair of legs of the fe- 

 male, and directs a strong current of water towards her by a rapid 

 rowing motion of the legs. The two spermatophores, which appear 

 on the outside, become liberated by this and are carried across to the 

 female, to which they become attached. The ovisacs form soon 

 after this (15 min.-l hour), taking but a few minutes for their forma- 

 tion. 



(d) Larvae. The young free-swimming larvae of Cyclops are called Nau- 



pliae, and are of theoretical importance from the occurrence of sim- 

 ilar larvae among widely different groups of Crustacea, thus furnish- 

 ing support for the theory that the Nauplias represents the primitive 

 Crustacean form. They are common in aqiiaria filled with females, 

 and are often seen in the field of the microscope during the study of 



