152 



LECTURE XIII. 



which must have grown after the head had become imbedded in the flesh 

 of the fish to which it is attached. Two long tubular processes or ex- 

 tremities are developed at the junction of the thorax with the abdomen ; 

 but their extremities are free, simple, slightly attenuated, and obtuse. 

 On the under surface of the body in the interspaces of these appendages 

 there are four pairs of simple, small, oval, flattened feet ; their pointed 

 extremities extend only half way to the sides of the part of the body 

 to which they are attached. The body is prolonged beyond the ovi- 

 sacs in the form of a tail, which is provided on each side with a sei'ies 

 of sixteen slender cylindrical appendages, close set in an oblique 

 position, like the barbs of a feather, or the vane of an arrow, whence 

 the specific name Sagitta, given to this parasite. The caudal la- 

 mellte of the higher Crustacea would seem to be here sketched out. 

 The anatomy of the Epizoa has been most elaborately traced out 

 by Nordmann in the parasite of the com- 

 mon perch, called Achtheres. In this 

 species two lateral teeth project from 

 the circular mouth, the labial margin of 

 which is fringed with bristles. Here 

 also we have mandibles and maxillae, the 

 latter provided with palpi ; and besides 

 these, a pair of jointed antennae {Jjg- 81. 

 a), each terminated by three setae. The 

 hepatic organ (e) is more concentrated 

 than in the Peniculus, and surrounds the 

 anterior part of the canal. The aliment- 

 ary canal is, as usual, straight, and ter- 

 minated by a bituberculate vent at the 

 opposite extremity to the head. The 

 abdominal intestine (^d) is fusiform, and, 

 divided by a series of slight constrictions^ 

 into sacculi. It is maintained in its po- 

 sition by a transverse muscle (/a). The 

 walls of the abdomen are distinctly pro- 

 vided with longitudinal (e) and transverse 

 (/i-) fasciculi of muscular fibres. The 

 nervous system consists of a single ce- 

 phalic ganglion, from which are conti- 

 nued two principal chords {g g) extend- 

 ing along the uTider surface of the body. 

 The circulating fluid consists of a 

 clear plasma, with granular corpuscles 

 The pulsatile vasiform heart may be 



Achtheres, female, maguilied. 



of different forms and sizes. 



