Miscellaneous. 497 



It is from not having had it at their disposal that the authors who 

 have investigated Pdtogaster (Rathke, Anderson, Lilljeborg, Koss- 

 mann, and so many others) have not succeeded in finding these parts. 

 In fact the absence of the digestive tube and of the limbs, and tho 

 want of determination of the cephalic and caudal extremities, de- 

 prive us absolutely, in the Centrogonida, of the marks which servo 

 to guide us in more regularly constructed animals. And how, 

 without reference-marks, is one to find an imperceptible ganglion, 

 lost in an innumerable mass of ova, each of which is at least twice 

 its size ? Thus I only succeeded in finding it in Saeculina after two 

 years of investigations. In Pdtogaster, on the contrary, although 

 the absolute difficulties of the search are exactly the same as in 

 Saeculina, I found it in the first individual dissected after less than 

 an hour's work. I mention this particular only to show the value 

 of the morphological method ; for if I have found this nervous 

 system it is by no means due to particular address in dissection ; it is 

 because, armed with the morphological data derived from the study 

 of Saeculina, I sought for it precisely where it ought to be found. 



The type Pdtogaster, although notably different from the type 

 Saeculina, may be regarded as derived from the latter in consequence 

 of certain modifications. The body is depressed and elongated ; the 

 mesenteric or ventral side * has diminished in length to the advan- 

 tage of the dorsal side, so as to carry the cloaca to one of the 

 extremities of the cylinder ; on the dorsal side a new mesentery has 

 been developed ; lastly, and this is the principal modification, the 

 cement-glands have ascended and, quitting the declivous parts of 

 the ovary, have gone to place themselves close to the peduncle and 

 the male sexual glands. 



In these displacements of the organs was the nervous ganglion 

 to retain its original situation at the bottom of the ovary, or was it 

 to follow the cloaca, or the mesentery, or the cement-glands ? Ob- 

 servation has shown that it did not remain immovable ; therefore 

 its relations with the declivous pole of the ovary are not at all essen- 

 tial ; it had followed the cloaca and the mesentery, but especially 

 the cement-glands, in their movements ; hence it is with these 

 organs, and chiefly with the last-named, that it has fundamental 

 relations. On the other hand, we see that the close relations of tho 

 ganglion with the testes in Pdtogaster are quite accidental, since 

 in Saeculina those organs are as far apart as possible. Hence- 

 forward in seeking for the nervous system in other Centrogonida, in 

 which the viscera may again affect new relations, we see that we 

 shall not have to pay any attention to the testes or to the anti- 

 peduncular pole of the ovary, and that it is between the cement- 

 glands, in the sagittal plane, and perhaps slightly towards the 

 cloaca and the cement-glands, that we must direct the forceps and 



* For the orientation of the animal, see the memoir cited, pp. 440 and 

 695. 



