THE COPEPODA 83 



free somite. There are three ostia, one median on the posterior 

 surface and a lateral pair. Anteriorly the heart gives oft' an aortic 

 vessel, usually very short. Only in Eucalanus is the aorta described 

 as extending into the frontal region of the head and dividing into 

 two pairs of lateral vessels. 



In the majority of the free-living Podoplea where the heart is 

 absent the blood is kept in motion mainly by the continuous 

 rhythmical backward and forward movements of the alimentary 

 canal, effected by the extrinsic muscles already mentioned. In 

 certain parasitic forms (Caligus, young Achtheres) a heart, or an 

 apparatus having an analogous function, is said to be present, but 

 exact details as to its structure are wanting. 



In Lernanthropus and some other Dichelestiidae a closed system 

 of vessels is present containing a yellowish or reddish fluid. There 

 is no heart, and the relation of this system to the circulatory 

 apparatus of other Copepoda is quite obscure. 



Excretory System. The maxillary ("shell") gland is the func- 

 tional excretory organ in the adult stage of most, if not all, 

 Copepoda. It is much larger in the freshwater forms, where the 

 duct is long and convoluted, than in the marine forms, in which it 

 is often hard to find, and sometimes apparently absent. The end 

 sac is small, and the tube terminates in a short chitin-lined duct 

 opening on the posterior surface of the maxilla. 



In the freshwater Harpacticid Belisarius a curious vibratile 

 organ is found connected with, or in close proximity to, the 

 maxillary gland. It has been supposed to be of the nature of 

 a " flame-cell," but it is more probably a muscular fibre or 

 membrane aiding the circulation of the blood in the neighbour- 

 hood of the gland. 



Glands. Unicellular dermal glands are present on the body and 

 limbs of most Eucopepoda. Certain pelagic forms belonging to 

 various genera of the Centropagidae and Oncaeidae are known to 

 be phosphorescent, and Giesbrecht has shown that this is due to 

 certain of the dermal glands, the secretion of which becomes 

 luminous on issuing from the apertures of the glands. In fresh- 

 water Cyclopidae and Harpacticidae the secretion of the dermal 

 glands envelops the body when the water dries up, and forms a 

 protective case enabling the animal to survive prolonged desiccation. 



Nervous System. The ventral nerve-cord is always short, not 

 reaching beyond the fourth free thoracic somite. It is divided 

 into distinct ganglia in the Gymnoplea, but in the Podoplea, so far 

 as is known, the ganglia are all coalesced. In the Corycaeidae and 

 in the parasitic families the whole system is still more concentrated, 

 forming a thick perioesophageal ring. Even in the Gymnoplea the 

 distinction between ganglia and commissures is not sharp, nerve- 

 cells being present abundantly on the latter as on the former. 



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