76' ME. W. S. EOWiSTEEE ON THE 



of the last gill-cleft. This organ, which has been described briefly by Kner * and in 

 detail by Sagemehlf, appears to be peculiar to the herbivoroiis forms, having been 

 recorded by these Avriters for Cnriinatm, Cceiwtropiis, Uemiodus, Citliarinus, and 

 Prochilodits ; and more recently by Boulcnger % for Xenocliarax. Discussing the 

 morphology of this curious organ, Sagemebl expresses the view that we have here to do 

 witli a structure arising from the gill of the rudimentary fifth branchial arch or 

 " inferior pharyngeal bone." If this conclusion be correct, the presence of the organ in 

 the herbivorous Characinids, and in these only, becomes of great interest, in view of the 

 fact that traces of a fifth gill have not been I'ound in any living Ganoid, but only in fishes 

 of yet lower organization — certain Selachians and Dipnoids. That this cliaracter was at 

 one time widely diffused is shown, as Sagemehl points out, by its presence in forms so 

 far separated as Notidanus and Protopterus. Two important conclusions seem to follow 

 from the identification of tlie organ under consideration with the gill of the fifth arch : 

 (1) that the herbivorous Characinids wliich possess it form a natural group ; aud (2) that 

 this divib.ion of the family cannot be derived from either the Erythrinoids or the other 

 carnivorous Characinids, but is at least as ancient as either of those groups. It should 

 be mentioned that an organ showing a somewhat similar minute structure has been 

 described by Hyrtl § and by Gegenbaur |I in certaiu Clupeids. 



Another structure — a rudimentary opercular pseudo-branch — is referred to by 

 Sagemehl ^ as being present in some Characinids, as also in Amia. The only forms in 

 which he mentions having found it are Citliarinus and the following carnivora — Hydro- 

 cyon, Anacijrtus, Alestes, and Tetrugonopterus. The Erytbrinoids show no trace of it, 

 contrary to what might have been expected from the consideration of their markedly 

 Amioid cranial characters. 



A Parasitical Crustacean. 



An Isopod crustacean, about 1'5 cm. in length, was found within the body-cavity of 

 a specimen of Anacyrtus microlepis from Asu.ncion, Ilio Paraguay. Mr. Stebbing, as I 

 have already mentioned, kindly identified it for me as Artystone trysihia **, family 

 Cymothoidse. I learn from him that the species is of some rarity, having been previously 

 only twice recorded, once from La Plata, and also from Brazil. My specimen, now in 

 the Biological Museum of the Royal College of Science, South Kensington, was enclosed 

 in a thick-walled capsule adherent to the ventral body-wall of the fish, and entirely 

 within tlie body-cavity. The rectum and adjacent parts were unfortunately cut before 



* Kiier, " Die Kiemenauhange dcr Characinidcn," VcrhancU. d. zoolog.-botanisohen Gesellsch. in Wicn, Bd. xi. 

 1861. 



+ Sagemehl, " On the Accessory Branchial Organ of Citliarinus" Morph. Jahrb., 1886-7. 



X Bouleiigcr, ' Les Poissons du Bassiu du Congo,' p. 199. 



§ HjTtl, '' Ueber die aceessorisohen Kiemcnorgane der Clupeaooen," Denkschr. d. k. Akad. zn Wien, Mathem.- 

 uatii'w. Klasse, Bd. x. 1855. 



il Gegenbaur, " Ueber das Kopfskclet v. Aleiioceitlmlus rostraius," Morphol. Jahrb., Bd. iv. 1878, Suppl. 



11 Sagemehl, 'Das Cranium dcr Characinidcn,' p. 113. 



** Schiiidte & Meinert, " Symbolje ad Monographiam C'ymothoarum : Pt. IV. Cymothoidse," 1883. Stebbing, 

 T. E. lu, •' History of the Crustacea," Intern. Soi. Ser., vol. Ixxiv. Cymothoidse, 1893. 



