50 



tiske Bevægelser, der med stor Eegelmæssighed for- 

 jjlantes langs ad dens hele Længde, ialmindeliglied 

 i Retningen hagfra fortil. 



CirculatioDSsystemet. — Hjertet danner (se Tab. 

 VI, Fig. 2, 3, 11 c) et overordentlig langstrakt, med 

 bestemte Mellemrum indsnoret Ror, der strækker sig 

 ovenfor Tarmen fra Nakkesegmentet og bagtil lige 

 ind i Halens næstsidste Segment. Det er ved fine 

 Muskeltraade fæstet til Indsiden af Ryggen.s Inte- 

 «•ument, og viser for livert Segment et Par venose 

 Spaltaabninger, hvorigjennem Blodet optages i Hjer- 

 tet. Den samlede Blodmasse, som paa denne Maade 

 trænger ind i Hjertet, ixdstodes dels fra den forreste, 

 dels fra den bågeste Ende, og kommer derpaa ind i 

 væglose Hulrnm mellem de forskjellige Organer. 

 Skjøndt saaledes et egentligt Karsystem, bestaaende 

 af Arterier og Vener, ganske mangler, circulerer dog 

 Blodet med stor Regelmæssighed omkring i Lege- 

 met, idet det følger visse bestemte Baner. Blodet 

 er, som sædvanligt, farvelost og indeholder en 

 Mængde smaa, tildels amøboide Blodlegemer, hvis 

 Gang let lader sig forfølge hos det levende Dyr 

 under Mikroskopet. Hjertets Pulsationer, hvorunder 

 Spaltaabningerne afvexlende aabnes og lukkes, er 

 særdeles livlige, saa det er meget vanskeligt at 

 tælle, hvormange der sker i Minuttet. 



Nervesystemet. — Centraldelene af Nervesyste- 

 met bestaar, som hos andre Crustaceer, af et dorsalt 

 Parti, det øvre Svælgganglion eller Hjernegangiiet, 

 og af en ventral Del, den saakaldte Buggauglie- 

 kjæde, begge forbundne ved en omkring Spiserøret 

 lobende Commissur (se Tab. VIII, Fig. 1). 



Hjernegangiiet (se Fig. 1, 4. 17), der har sin 

 Plads i den forreste Del af Hovedet, lige foran Be- 

 gj-ndelsen af Tarmen, er af temmelig betydelig Stør- 

 relse, og bestaar af 2, i Midten med hinanden for- 

 bundne symetriske Halvdele. (Jventil danner det 

 flere Lappe, der delvis omfatter Basis af det enkle 

 Øie (se Fig. 4), og til hver Side fortsætter det sig 

 i den mægtigt udviklede Synsnerve (o), etterat have 

 udsendt en tynd Nerve til Iste Par Antenner (a'). 

 Et andet Par Nerver sees at udgaa fra Hjernegang- 

 iiet længere fortil, til hver Side af det enkle Øie. 

 De ender hver med en liden ganglios Opsvulmning 

 umiddelbart under en liden grubeformig Fordybning 

 i Hovedets forreste Integument (a), rimeligvis sva- 

 rende til de omtrent paa samme Sted hos Lrinnefis 

 forekommende eilierede (irruljer, der ialmindeliglied 

 ansees for et Slags Sandseorgan. Nerverne for 2det 

 Par Følere (a''), der hos Hannerne er betydelig 

 stærkere end hos Hunnerne, udspringer fra den for- 

 reste Del af Svælgcommissurerne. Disse sidste for- 

 bindes, inden de omfatter Spiseroret, ved en tynd 

 Tværeommissur (se Fig. 1 og 4), der ligger ved Basis 

 af ( )verlæben. 



be observed, these being transmitted with great 

 regularity throughout its entire length, generally 

 from behind forwards. 



Circulatory system. — The heart (see PI. VI, 

 figs. 2, 3, 11 c) consists of an exceedingly elongated 

 tube, constricted at regular intervals, and extending 

 above the intestine from the cervical segment back- 

 wards into the penultimate segment of the tail. It 

 is attached by fine muscle-fibres to the in.side of the 

 dorsal integument, and exhibits, in each segment, 

 a pair of venous ostia, through which the blood 

 is received into the heart. The accumulated mass 

 of blood, which thus forces itself into the heart, is 

 expelled partlj^ from the anterior, partly from the 

 posterior end, and then enters the several cavities 

 between the various organs. Although there is thus 

 a total absence of a true vascular system consisting 

 of arteries and veins, yet the blood circulates with 

 great regularity through the body, following certain 

 fixed courses. The blood is, as usual, colourless, 

 and contains a number of small, partly araæbous 

 blood-corpuscles, whose course may be easily traced 

 imder the microscope. The piilsations of the heart, 

 during which the ostia alternately open and close, 

 are exceedingly rapid, so that it is very difficult to 

 count the number occuiTing in a minute. 



Nervous system. — The central portion of the 

 nervoiis system consists, as in other Crustaceans, 

 of a dorsal part, the supraoesophageal or cerebral 

 ganglion, and of a ventral part, the so called ventral 

 ganglion chain, connected with one another by a 

 commissure round the æsophagus (see PI. VIII. fig. 1). 

 The cerebral ganglion (see tigs. 1, 4 and 17>, 

 which is situated in the anterior part of the head, 

 just in front of the commencement of the intestine, 

 is of considerable size, and consists of 2 sj-mmetri- 

 cal halves connected with one another in the middle. 

 Above, it forms several lobes which partially sur- 

 I'ound the base of the ocellus (see fig. 4), and, after 

 sending out a fine nerve to each of the fir.st pair of 

 antennae (a'), continues on each side in the form of 

 the powerfully developed optic nerve (o). A second 

 pair of nerves are seen to issue from the cerebral 

 ganglion farther forward, on each side of the ocellus. 

 They each end in a little ganglionic protuberance 

 immediately below a small pit-like depression in the 

 anterior integument of the head (a) probably an- 

 swering to the ciliated pits occurring in about the 

 same place in Limnetis, and which are generally 

 considered to be a kind of organ of sense. The 

 nerves for the second pair of antennæ (a^) which 

 are considerably stronger in the males than in the 

 females, originate from the anterior part of the 

 æsophageal commissures. These, before they encircle 

 the æsophagus, are connected by a thin transverse 

 commissure (see figs. 1 and 4) lying near the base 

 of the anterior lip. 



