REVIEW OF GENUS ARGULUS — MEEHEAN 465 



above, the opening is oblong or round and approximately the length 

 of the fold, usually with two elongate papillae forming a groove per- 

 pendicular to the axis of the appendage and directed toward the open- 

 ing. The opening is shaped and placed according to the structure and 

 shape of the peg on the next appendage. 



On the interior of the pocket are three folds, of which the median 

 one is modified for the accommodation of the size of the pocket to 

 lit the peg and according to the size of the appendage of the female 

 that is clasped by this apparatus. There are other types of sockets, 

 which will be described in connection with the species concerned. 



The peg on the fourth appendage is located on the distal edge of 

 the anterior surface of the basis near the base of the exopod (fig. 34, d). 

 It is variously shaped according to the species. Generally it is some 

 variation of a pear-shaped structure with a narrow tip turned later- 

 ally. On the proximal side and ventrally there is a chitinous ridge 

 around the base of the peg extending obliquely across the appendage. 

 It may be covered with very short spines or entirely unarmed. 



The peg and socket apparatus is used entirely as a clasping medium. 

 The last appendage of the female is clasped from the dorsal side, 

 and the peg is fitted in the socket, which is enabled to accommodate 

 itself by the folds already described. The accessory organs on the 

 other appendages assist in this process. The abdomen of the male is 

 twisted around that of the female first on one side, then on the other, 

 so that the ejaculatory duct, located ventrally between the legs at the 

 posterior end of the thorax, is brought directly in contact with the 

 spermathecae of the female. These are located on the under side of 

 the abdomen close to the thorax. 



The testes are a pair of elongate bodies lying on each side of the 

 midline in the abdomen. In the female the ovaries lie in the thorax 

 and the spermathecae appear as small rounded bodies in the anterior 

 end of the abdomen, often with a pair of tactile papillae projecting 

 posteriorly from near the opening. 



Two respiratory areas are located on the ventral surface of the 

 carapace lateral to the body. The smaller of these may be anterior 

 to the larger or may be mesial to and partially surrounded by it. 

 Their function is considered as respiratory, because it has been found 

 that there are blood sinuses under them limited by a single layer of 

 hypodermal cells. In most instances these areas have a character- 

 istic shape for a given species. 



The anterior portion of the carapace and a portion of the lateral 

 edges are flattened to fit against the body of the fish. These portions 

 are armed with small spines directed posteriorly. The various ventral 

 portions of the body of the animal that may come in contact with the 

 body of the fish are also armed in the same manner. 



