408 CRUSTACEA. 



ORDER SIPHONOSTOMATA. 



Genus Akgulus. 

 A.fuliaceiis, Juriiie. 



Belfast, Oct. 29, 1838. — In our market to-day I had the pleasure of 

 detectinpj one of these very interesting and handsome parasites attached 

 to the dorsal fin of a Salmo Trutta, about a foot in length. The Aryiihis 

 is 3i lines long, is a female, and in addition to the ova exhibits at the 

 base of the tail the dark green spots (" noirs," Desm. Consid. Gen. Crust., 

 p. 332) Avhich are considered to mark this sex. Although the fish to 

 which it was attached had been for some hours out of the water, the Arffu- 

 lus held so firmly by its two disks that I had some difficulty in detaching 

 it without injury. For about ten minutes it was Avrapped in a piece of 

 dry paper, and then placed in a vessel of water in which salt had been 

 dissolved until it was to the taste like strong sea-water.* This was no 

 sooner done than my pretty captive, after drawing her last pair of feet 

 together several times,i- thus calling to mind the common housefly, struck 

 out her oai's, and thereby was rapidly impelled through the fluid. 



The figures of Desmarest (tab. 50) and Yarrell (Brit. Fish., vol. ii. p. 

 399) are very characteristic of this species, but the great beauty exhibited 

 in the specimen before me is at the same time not shoMu, perhaps in con- 

 sequence of the upper side of the female not being represented — this con- 

 sists in its being closely spotted with very dark green along the central 

 part of the body for two-thirds posteriorly, commencing a little above the 

 ovary in the form of a head, and extending to the posterior portion ; the 

 rest of the upper side of the body being of a very pale yellowish green hue 

 and semi-transparent as described, the part thus spotted is well defined, 

 and is strikingly of the form of a coleopterous insect, which the Arf/idus 

 in another point of view resembles, when the two sides of the greenishly 

 transparent " boucliers " are thrown a little apart, as we see the elytra of 

 the insect. I was further reminded of the resemblance when attempting 

 to remove it, as in holding firmly by the suckers, the body was drawn in, 

 and the " boucliers " elevated quite above it. Its motion through the 

 water seems equally rapid, whether it be on the upper or under side, or 

 swimming retrally — it frequently moved along the surface with its back 

 downwards, and was wholly immersed except the suckers, which were 

 thrown either on a line with the water or quite above it, and thus would 

 the animal occasionally remain quiescent for a short period. 



The constant motion of these organs (visible to the naked eye) in addi- 

 tion to the rapid ])lay of the feet, impart much life to the appearance of 

 the Argulus, and present not the same aspect for two continuous seconds 

 of time, whether the body be at rest or otherAvise. They — i. e. tlie mar- 

 ginal row of minute suckers, Avhich appear as a dark line round the 

 disk in figures of the species — -are frequently drawn together to the 

 centre of the disk, exhibiting a dark point not larger than the eye. 



* This was done in consequence of my having been told that the fish was 

 taken in the sea ; the stomach, however, contained the remains of fresh-water 

 insects (according to my friend A. H. Haliday, Esq., to whose inspection they 

 were submitted), which possibly might have been washed into the sea and there 

 obtained, biU this is by no means probable. 



t I observed this repeatedly done afterwards — they seem to be rubbed against 

 the caudal plates. 



