352 THE INVERTEBRATA 



any of the appendages. He may also possess organs for the transfer- 

 ence of sperm: these may be modified appendages or protrusible 

 terminal portions of the vasa deferentia. The gonads of both sexes 

 (Fig. 234) are hollow organs from which ducts lead directly to the 

 exterior. Primarily there is one gonad on each side, but they often 

 unite more or less completely above the alimentary canal. The ducts 

 usually open near the middle of the body, though the male openings 

 of cirripedia and some cladocera are almost terminal and the female 

 opening of cirripedia is on the first thoracic somite. Save in the Cirri- 

 pedia, the Malacostraca, and some of the Cladocera, the ducts of the 

 two sexes open upon the same somite. 



The spermatozoa are very varied in form and often of complex 

 structure ; usually, but not always, they are immobile. They are trans- 

 ferred to the female, often in packets (spermatophores) . The ova have 

 usually much yolk, and meroblastic, centrolecithal cleavage (Fig, 



Fig. 235. A ventral view of the first Nauplius of Cyclops. After Dietrich. 

 an.' antennule; an." antenna ;^n. gnathobase; Ibr. labrum; md. mandible. 



213 A-C), but sometimes are less yolky and undergo total cleavage. 

 Gastrulation may be by invagination (Fig. 213 D) or by immigration. 

 Occasionally the eggs are set free at laying, but in the great majority of 

 cases they are retained for a time by the mother, either in some kind 

 of brood pouch or adhering in some way to her body or appendages. 

 Development is not infrequently direct, but in most cases involves a 

 larval stage or stages. 



Typically, the crustacean hatches as a Nauplius larva (Fig. 235), a 

 minute creature, egg-shaped with the broad end in front, unseg- 

 mented, but provided with three pairs of appendages — the antennules, 

 which are uniramous, and the antennae and mandibles, which are 

 biramous and should each bear a gnathobasic process or spine 

 directed towards the mouth, though those of the mandibles are often 

 not developed at first. The antennal ganglia are as yet postoral (see 

 p. 340). The median eye is the only organ of vision. A pair of frontal 

 organs (p. 342) are present as papillae or filaments. There is a large 



