314 THE INVERTEBRATA 



ostia; in most of the Cladocera it is a sac (Fig. 233, h!) with only one 

 pair. In the Cirripedia and many of the Copepoda and Ostracoda 

 the heart is absent and the blood is kept in movement only by the 

 movements of the body and alimentary canal. In the parasitic copepod 

 Lernanthropus and some related genera there is a remarkable system 

 of closed blood vessels v^ithout a heart. 



The blood is a pale fluid, which bears leucocytes except in ostracods 

 and most copepods. It contains in the Malocostraca the respiratory 

 substance haemocyamn, in which copper is present in combination 

 with a protein. In various entomostraca, notably in Lernanthropus ^ 

 just mentioned, haemoglobin has been found. 



As is usual with animals that are free and active, the sexes are 

 separate in the great majority of the Crustacea, though the Cirripedia, 

 which are sessile, certain of the parasitic Isopoda, and a few excep- 

 tional species in other groups, are hermaphrodite. Parthenogenesis 

 takes place in many of the Branchiopoda and Ostracoda, and in these 

 it is often only at more or less fixed intervals that sexual reproduction 

 occurs. The male is usually smaller than the female and in some 

 parasites is minute and attached to her body. He has often clasping- 

 organs for holding his partner, and these may be formed from almost 

 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. 223) 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 

 Cirripedia, 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. 202), 

 but sometimes are less yolky and undergo total cleavage. Occasionally 

 they 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. Develop- 

 ment is not infrequently direct, but in most cases involves a larval 

 stage or stages. 



Typically, the crustacean hatches as a Nauplius larva (Fig. 224), 

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

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



