Fig. 218. Lerneans.

* Muller (O. F.), Zoologia Danica, 1788.

(1084). These examples, however, are taken from the most imperfectly 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 Achtheres Percarum (fig. 219) is one of those species most nearly allied to the Articulata; 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). The Achtheres 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 considerably smaller.

(1088). The outer covering of the body of these little creatures is at once seen to have assumed a horny hardness approximating to the density of the coverings of the Entomostraca; and indications 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 Achtheres fixes itself to the gums of the fish. The hinder pair of extremities alluded to (fig. 219, b b) are, in fact, enormously developed; they curve forward after their origin from the posterior part of the trunk, and are so much extended that they project considerably beyond the head of the creature, where, becoming considerably attenuated, the two are joined together by a kind of suture, and support, upon the point where they are united, a cup-shaped organ whereby the creature fixes itself. This singular instrument (represented upon an enlarged scale at fig. 220, l) is of a cartilaginous hardness, resembles a little bowl the inside of which is studded with sharp teeth, and is not only calculated to act as a powerful sucker, but, from the hooks within its cavity, is capable of taking a most tenacious hold upon the lining membrane of the mouth.

Adheres Percarum: a, adhesive disk; b b, posterior pair of limbs; e, stomach; d d, ovaria; e, anal orifice; ff, ovisacs; o, antennae.

Fig. 219. Adheres Percarum: a, adhesive disk; b b, posterior pair of limbs; e, stomach; d d, ovaria; e, anal orifice; ff, ovisacs; o, antennae.

* Mikrographische Beitrage zur Natiirgesehichte der wirbcllosen Thiere. Berlin, 1832.

(1090). The other members (fig. 219, o) are much less developed, but are nevertheless so constructed as to assist materially in fixing the Epizoon; they are represented upon a very large scale in fig. 220, 2, where the outer pair (a a) are seen to exhibit, in the transverse lines indented upon their surface, the first indication of articulated limbs, and their extremities, armed with minute hooks, evidently form powerful agents for prehension. Internal to these are two other jointed organs, still more feeble in their construction, the ends of which (b b), being armed with three spines, will assist in effecting the same object.

(1091). The mouth itself (fig. 220, 2, c) is formed upon similar principles, the external orifice being surrounded with a circle of minute recurved spines, well calculated to ensure its firm application to the surface from which nourishment is obtained; and within this, rudimentary jaws furnished with strong teeth are visible, adapted, no doubt, to scarify the part upon which the mouth is placed, in order to ensure an adequate supply of food. In the male Achtheres, the sucking-bowl possessed by the female does not exist, the prehensile organs being merely four stout articulated extremities, armed at the end with strong prehensile hooks.

Details of the structure of Achtheres. 1. Adhesive disk on an enlarged scale.

Fig. 220. Details of the structure of Achtheres. 1. Adhesive disk on an enlarged scale: e e, conjoined extremities of the hinder pair of legs. 2. Structure of the mouth and parts adjacent: a a, anterior limbs; b b, antennae; c, mouth, furnished with rudimentary jaws.

(1092). As we might suppose, from the nature of the food upon which this creature lives, the alimentary system is extremely simple. The oesophagus (the course of which is represented by dotted lines in the same figure) terminates in a straight digestive canal (fig. 221, a), which passes through the centre of the abdomen; but no separation between stomach and intestine is visible. The entire tube, from the transverse constrictions visible upon its surface, has a sacculated appearance, and is perceptibly dilated towards the centre of the abdominal cavity; after which it again diminishes in size as it approaches the anal orifice (6), situated at the posterior extremity of the body.

(1093). Near the termination of its course, the alimentary canal passes through a loop formed by transverse bands (n n), and, moreover, seems to be retained in its position by radiating fibres, apparently of a ligamentous character, but which have been described as representing a biliary apparatus.