PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 289 



in which also it is bow-shaped, with the concavity forward), and through 

 still another commissure with the two upper pharyngial ganglia. 



From each inferior pharyngial ganglion, a long, thin commissure ex- 

 tends, binding it with a large subovate ganglion, which may be called 

 the posterior inferior pharyngial ganglion; and the others must then 

 take the name of the anterior inferior pharyngial ganglia. The two 

 former are connected by a little arched commissure, and lie behind the 

 aorta. From the anterior inferior pharyngial ganglia three pairs of 

 nerves proceed before and one behind the buccal muscle. The anterior 

 superior pharyngial ganglia are connected by five commissures with 

 each other and other ganglia. From the superior anterior pharyngial 

 ganglia proceed two small nerves for the upper buccal muscles. The 

 posterior superior pharyngial ganglion sends out a small nerve to the 

 upper lateral buccal muscle, and from the superior posterior post- 

 pharyngeal two small nerves are traceable to the radula. 



Circulation. — Our knowledge of this is due to Middendorf, to whose 

 ponderous and not very satisfactory monograph of Cryptoehiton Stelhri 

 the student is referred. More light is needed on this subject. 



Sexual Organs. — The Chitonidcc are of two sexes, wherever they have 

 been examined by the writer, and the number of forms which has passed 

 under review is so large that there can be no doubt this is the rule 

 throughout the group.* The histological characters of the male and 

 female gland resemble those of the Limpets, at least in general appear- 

 ance. The most superficial observer can separate the sexes when the 

 characters have once been called to his notice. It is true that Midden- 

 dorf found, or believed he found, spermatozoa in the ovisac of Chiton 

 (Symmetrogej>hyrus) Pallasii, but this may be accounted for in another 

 way; and I may say, definitely, that I have examined both males and 

 females of that species. The glands of both sexes open on each side of 

 the anus, in some species quite close to the latter, in others much further 

 forward and in advance of the most posterior branchiae. The opening 

 may be a simple pore or small aperture forming the termination of a 

 sexual duct, or it may consist of what I have termed a fenestra, or 

 elongated slit, crossed by several bands of tissue, so that there may 

 seem to be from two to seven oblique slits, each extending partly behind 

 the front end of the slit behind it. In these cases, I have not been able 

 to determine the existence of a continuous oviduct, and am* inclined to 

 believe that the ova may pass from the oviduct into the perivisceral 

 cavity, and from thence, through the fenestra?, reach the exterior. 



The ovisac and spermsac are more or less convoluted and asymme- 

 trical. They are probably the result of fusion of two original glands in 

 the median line, if, indeed, they are not partly separated in some 1 spe- 

 cies, as seemed in one or two instances to be the case. The ducts, when 

 carefully examined, are seen to spring from the anterior abdominal side 

 of the sac, not from the posterior end. 



* Dr. v. Ihering arrives at the same conclusion. 



Proc. Nat. Mus. 78 19 Feb. 13, 1879. 



