Order 5. ISOPODA. 431 



observations on the circulation of the Isopoda, and especially in the Ligiae. The heart has 

 the form of a long vessel, extended above the dorsal face of the intestine ; from its anterior 

 extremity are emitted three arteries, as in the Decapods, but from their examination it would 

 seem that the venous system is not so complete as in the Macroura. In respect to the 

 nervous system, there are nine ganglions, not including the brain, but the two anterior and 

 the tvi^o posterior are so nearly together that they may be reduced to seven. The second and 

 six following send forth nerves to the legs, and the tail is furnished with nerves fiom the last 

 ganglion. 



The females carry their eggs underneath the breast, either defended by scales, or in a pouch 

 or membranous sac, which they open in order to allow the young ones to escape ; these are 

 born with the form and parts peculiar to their own species, and merely increase in size by 

 changing their skins. [M. Milne Edwards, in his interesting " Observations sur les change- 

 mens de forme que divers Crustaces eprouvent dans le jeune age," (published in the Annales 

 des Sciences Naturelles,) has given a detailed account of the peculiarities which distinguished 

 the young individuals of Cymothoa trigonocephala and Anilocra mediterranea, which had been 

 extracted from between the pectoral plates of the females. In the newly-hatched young, the 

 tail is longer and narrower than in the perfect animal, and it has only six thoracic segments 

 and six pair of legs.] 



The greatest number of the species reside in water. Those which are terrestrial have like- 

 vnse need, as is the case with other Crustacea living out of the water, of a certain degree of 

 atmospheric humidity, in order to enable them to respire, and keep their branchiae in a state 

 fitted for that function. 



This order, in the system of Linnaeus, consists of the genus 



Oniscus, — 

 which we distribute into six sections. 



The first section, Epicardes, Latr., is composed of parasitic Isopods having neither eyes nor antennae, 

 of which the body is very flat, small, and oblong in the males, but much larger in the females, of an 

 oval form, narrow and rather bent posteriorly, concave beneath, with a thoracic rim, divided on each 

 side into five membranous lobes, the legs being inserted on this rim, very small and bent round, and 

 fit neither for crawling nor swimming ; the under-side of the tail is furnished with five pairs of small 

 ciliated imbricated plates, answering to the same number of segments, and arranged into two longi- 

 tudinal rows, but the posterior extremity of the body is not furnished with appendages. The mouth 

 only distinctly exhibits two membranous plates, applied upon another of the same consistence, being 

 of a quadrilateral form. The hollowed part of the body is filled with eggs, and near the situation 

 where they are discharged the presumed males are constantly found, but their exceedingly minute size 

 seems to render the act of coupling impossible. These Crustacea form only a single subgenus, — 



Bopyrus, Latr., the common and typical species of wliich is the Bopyrus crangorum, Fab., which is parasitic 

 upon the Common Prawns, Pal<emon sqidlla and serratus, affixing; itself beneath the carapax, upon the branchiae, 

 when it produces on the side of tlie body attacked a tumour or swelling' like a lens. The fishermen of La Manche 

 believe that these parasites are young soles, [to which fish they bear a slight resemblance in form]. 



M. Risso has described a second species [B. Palemonis, Risso, Crust. Nice. p. 148], beneath the body of the 

 female of which he observed between eight and nine hundred minute young ones, [easily visible with a lens, of a 

 greyish white colour, and which the parent has always the instinct to deposit in the places frequented by the 

 Palsemons ; and as soon as the young are free they attach themselves to their prey]. 



The second section, Cymothoada, Latr., comprises those Isopoda which have four distinct antennae, 

 setaceous, and ordinarily terminated by a multiarticulate filament, having eyes and a mouth composed 

 of the ordinary parts (see the general observations upon the Malacostraca Edriopthalma), and vesicular 

 branchiae disposed longitudinally in pairs. The tail is composed of four or six segments, with a swim- 

 ming plate on each side near the tip, and the five legs are generally terminated by a strong hook or 

 claw. All the Cymothoada are parasites. 



In Serolis, Leach, the eyes are placed upon tubercles on the back of the head, and the tail is composed of only 

 four segments. The antennae are arranged in two lines, and terminated by a multiarticulate filament. Beneath 

 the three basal segments of the tail, between the ordinary appendages, there are three others, transverse, anii 

 terminated posteriorly in a point. One species was only known [tc Latreille, nafaely, the Cymothoa par adoxa, Fab. 



