ISOPODA. 431 



observations on tlie circulation of the Isopoda, and especially in the Ligiic. Ihc heart has 

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

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

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

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

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

 six lollowing send forth nerves to the legs, and the tail is furnished with nerves from 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 ivith 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 Us chanye- 

 mens de forme que divers Crustaces eproiwent dans le jeune dge," (published in the Annates 

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

 the young individuals of Cymottioa trir/onoceptiala and Anitocra 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- 

 wise 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 branchia; in a state 

 fitted for that function. 



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



Oniscus, — 

 which we distribute into six sections. 



The first section, Epicaudes, Latr., is composed of parasitic Isopods having neitlier eyes nor antennie, 

 of wliicli 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 lulies, 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 imlirieated plates, answering to the same number of segments, and arranged into two longi- 

 tndiiial ro^^■s, l)ut tlie posterior exti-emity of the body is not furnished with appendages. The month 

 only distiiiefly exhibits two membranous plates, applied upon anotiier of the same consistence, being 

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

 ^vbere 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, — 



Bop'/nis, Latr., the common and typical species of wllich is the Bopyrus crangorum^ Fab., which is parasitic 

 upon the Common Prawns, Pal.cmon .squHla and serratus, atfixini; itself beneath the carapax, upon the branchia;, 

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

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



M. Risso has described a second species [it. Piilemonis, Risso, Cntst. Nice. p. US], beneath the body of the 

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

 2:reyish white colour, and which the parent has always the instinct to dejiosit in the places frequented by the 

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



The second section, Cymotho.xda, Latr., comprises those Isopoda which have four distinct antenna, 

 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 

 branchia: 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 Cyniottioada are parasites. 



In Scrali.-i, Leach, the eyes are placed upon tubercles on the back of the head, and the tail is composed of only 

 four segments. The antenna? are arran^^ed 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, auil 

 terminatei' posteriorly in a puuit. One species was only known [to Latreille, namely, the Cymottioa paradaxa. Fab. 



