ACHTHERES PERCARUM. 



423 



tenacity of its hold upon the surface from which it imbibes food. The 

 sacculi appended to the posterior part of the animal are receptacles for 

 the eggs and will be explained hereafter. 



(1084.) These examples, however, are taken from the most imper- 

 fectly organized Epizoa; but as we ascend to more highly- developed 

 species, we shall at once see how gradually an approximation is made to 

 the articulated outward skeleton and jointed limbs met with in the 

 Homogangliate forms of being, until at last the zoologist remains in 

 doubt whether the more elaborately-constructed ought not to be admitted 

 among the Crustacean families which they most resemble. 



(1085.) The Achiheres Percarum (fig. 219) is one of those species most 

 nearly allied to the AHTICTJLATA ; and 

 the details of its anatomy having been 

 fully investigated by Nordmann*, it 

 will serve as a good example of the type 

 of structure which prevails throughout 

 the group. 



(1086.) TheAchtTieres is found to infest 

 the Perch (Perca fluviatilis), adhering 

 firmly to the roof of the mouth, to the 

 tongue, or sometimes even to the eyes 

 of that fish, in which situations it is 

 concealed by a brownish slimy secretion, 

 so that its presence might easily escape 

 the notice of a casual observer. 



(1087.) The female, represented in 

 the figure, is about 2 lines in length ; 

 the male, which differs materially from 

 the other sex in many points, is con- 

 siderably smaller. 



(1088.) The outer covering of the body 

 of these little creatures is at once seen 

 to have assumed a horny hardness ap- 

 proximating to the density of the cover- 

 ings of the Entomostraca ; and indica- 

 tions are even perceptible of a division into segments : the distinction^ 

 moreover, between the trunk (cephalothorax), to which the limbs are 

 appended, and the abdomen, wherein the viscera are lodged, is obvious. 



(1089.) Instead of the rude and imperfect limbs we have seen in the 

 Lerneans, the legs are visibly more perfect in their entire construction ; 

 and in the female, the posterior pair of these appendages are converted 

 into a most singular instrument of attachment, whereby the Acliiheres 

 fixes itself to the gums of the fish. The hinder pair of extremities alluded 



* Mikrographische Beitrage zur Natiirgeschichte der wirbcllosen Tliiere. Berlin, 

 1832. 



Adheres Percarum: a, adhesive 

 disk; 6 6, posterior pair of limbs ; 

 c, stomach ; d d, ovaria ; e, anal ori- 

 fice ; ff, ovisacs ; o, antennae. 



