CHOXDRACAXTHUS GIBBOSUS. 241 



seems to be connected with a subjacent ganglionic mass. 

 The body is sufficiently transparent to allow the pulsations 

 of a heart to be seen, but none can be discovered. The testis 

 is a large oval bilobed mass (£), lying like a saddle upon the 

 anterior part of the intestine. From this body a thick vas 

 deferens runs back upon each side of the intestine, and di- 

 lates in the penultimate and antepenultimate somites into a 

 thick walled pyriform sac — a sort of vesicula seminalis. The 

 embryo leaves the egg as a Nimplius, like that of Cyclops. 



There are many genera of these parasites, some of which, 

 such as the almost completely vermiform Jjerncece, deviate 

 even more widely than C ho ndr acanthus from the ordinary 

 form of Crustacea, while others, such as Ergasilus and JYoto- 

 delphys, differ but little from the free Copepoda. 



In Caligus, the labium and metastoma are elongated and 

 united into a tube in which the sharp styliform mandibles are 

 inclosed ; and from the prevalence of this suctorian form of 

 mouth in some of the best known species of parasitic Cope- 

 poda, they are frequently termed "suctorial" crustaceans. 

 Suctorial disks for attachment are developed from the coa- 

 lesced posterior pair of thoracic members in Achtheres ; and, 

 in this genus, the head, as a distinct part, becomes almost 

 entirely obsolete. 



Argulus, the parasite so common on the Stickleback, is 

 worthy of notice as one of the most curious modifications of 

 the epizoic type. 1 It is extremely flattened, and is composed 

 of an anterior cephalo-thoracic disk, behind which lies a very 

 short and broad, notched abdomen. A median styliform 

 weapon lies in a sheath in front of the mouth, and the small 

 mandibles and maxillae are inclosed in a short tube formed 

 by the labrum and the metastoma. Six pairs of appendages 

 lie behind the mouth, the anterior being metamorphosed into 

 suckers, the next pair into strong limbs with a toothed sec- 

 ond joint, and the four others constituting biramous swim- 

 ming-feet. There are two pairs of antennary organs, and tw r o 

 compound eyes. According to Leydig, the males are pro- 

 vided with cups on their penultimate swimming-feet ; and, 

 during copulation, these are filled with the seminal fluid, 

 which is thus transferred to the vulva of the female, and 

 thence to the spermatheca. The eggs are laid, and not car- 

 ried about in ovisacs. The larva is provided with two pairs 



1 Claus (" Ueber die Entwickelunar, Organization und systematische Stellung 

 der Arsruliden," 1875) has proved the close affinity of Argulu* with the Cope- 

 poda., but proposes to regard it as the type of a special group, the Branchiura. 



