TYPE CRUSTACEA. 395 



TecTiiction, wtile the second pair is frequently adapted to form 

 organs for adhering to the host, and finally the thoracic 

 appendages may undergo various stages of degeneration, in 

 some forms entirely disappearing. 



Branchial organs are entirely wanting throughout the 

 order, respiration taking place over the entire body surface. 

 A heart is present in a few forms (Calanidse) consisting of a 

 saclike organ with but a single pair of ostia, but in the 

 majority of cases it is wanting. A single median eye is gen- 

 erally present, and in a few forms, Pontella, Gorycceus, and 

 Argulus, lateral eyes are also present, though absent as a 

 rule throughout the group. Each lateral eye in Gorycceus 

 consists of a single ommatidium, but in Argulus is compound 

 and similar to the lateral eyes of the Branchiopoda. 



The Copepoda are throughout bisexual even in the cases 

 of the parasitic forms. The vasa deferentia are provided with 

 an enlargement in which the spermatozoa are included within 

 a capsule, forming a spermatophore which during copulation 

 is deposited in the neighborhood of the female genital open- 

 ing. The spermatozoa being discharged from the spermato- 

 phore-capsule, by a special discharging apparatus with which 

 it is provided, make their way into a receptaculum seminis 

 which communicates with each oviduct, the ova being fertilized 

 during their passage to the exterior. These are usually 

 carried in one or two masses attached to the first abdominal 

 segment of the female, though in some forms, such as Notodel- 

 phys, they undergo their development in a brood-chamber 

 formed by the duplication of the integument of the dorsal 

 surfaces of the fourth and fifth thoracic segments. A peculiar 

 dimorphism of the sexes occurs in some of the most highly 

 modified parasites, such as Chondr acanthus, Achtheres, and 

 otliers, the male being very much smaller than the female 

 and showing much less degradation, frequently presenting 

 well-developed eyes and more or less perfectl^'^-developed 

 appendages, so that it is able to lead for a time a free exist- 

 ence. It is to be regarded as a larval stage sexually mature, 

 since it resembles closely the female when in the stage immedi- 

 ately before fixation to its host, the greater part of the degen- 

 eration taking place after that has been accomplished. 



Two suborders are recognizable. 



