272 



LECTURE XIII. 



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 

 lamellae 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 common 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 {b), and maxillge, the latter provided with palpi ; and, be- 

 sides these, a pair of jointed antennae (c), each terminated by three 

 setae. Now, this is a very important external character ; it is the first 

 instance of true jointed antennae that we have met with in our ascend- 

 ing survey of the animal kingdom, and the acute zoologist might be 

 led to surmise, from their presence, that 

 relations of higher affinity were masked 

 beneath the general character of the ver- 

 miform body of the parasite, and we shall 

 be able to raise the mask as we trace the 

 metamorphoses of the species. The ali- 

 mentary canal is, as usual, straight, and 

 terminated by a bituberculate vent {y), 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 posi- 

 tion by a transverse muscle (Ji). The he- 

 patic organ (e) is more concentrated than 

 in the Peniculus, and surrounds the ante- 

 rior part of the canal. The walls of the 

 abdomen are distinctly provided with lon- 

 gitudinal (i) and transverse (k), fasciculi 

 of muscular fibres. The nervous system 

 consists of a single cephalic ganglion, from 

 which are continued two principal chords 

 (g), extending along the under surface of 

 the body. 



The circulating fluid consists of a clear 

 plasma, with granular corpuscles of diffe- 

 rent forms and sizes. Tlie pulsatile vasi- 

 form heart may be seen at the middle line 

 of the cephalo-thorax propelling the blood 

 forwards by rythmical contractions. Two canals {n), pass from it 

 into the hollow prehensile feet. The rest of the blood is distri- 



* LXXI. Ilcft. ii. 



Achtheres, female, magnified. 



