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Show 126 CRUSTACEA. the anterior and inferior extremity of the head which have but a single joint with one or two setce at the extremity. In the PoLYrHEMtrs, Mull. As in Daphnire and Lynceus, the antennre are in the form of d' · d d · oars IVl e mto two branches; but each of them is composed of five JOints. The head, moreover, which is very distinct and rounded • 'd d ' h f ' 1 prov1 e Wit a sot·t o neck, and IS almost entirely occupied by IaI large eye. The feet ai•e completely exposed. But a ~ingle species has hitherto been discovered, the Mono. culusped~culus, L.; Deg., Insect., VII, xxviii~ 6-13; Polyphemus oculu~, Mull., Entom., xx, 1-5; Cephaloculus stagno1·um, Lam.; Jur1ne, Monoc., xv, 1-3; Des mar., Consid. LIV 1 Gl The t ee t , accor d'm g to J urt· ne, have no resemblance w' hatev' er' t"o· the Monoculi of this divis.i o.n . They consist of a thigh ' le g, an d a tarsus composed of two JOints, from the extremity of which th of the last pair excepted, issue several small threads. Two ~ alii ant ennre, cons.i st.m g o f a single joint and terminated by two th rnda · f rea s, proJect ro~ the anterior extremity of the head. The shell is 80 extt·emel~ diaphanous, that all the viscera can be distinguished. :rhe ~atr1x, w~en filled with eggs, occupies the greater part of its mtenor. Their greatest number never exceeds ten. In following the gradual development of the fretus, we are struck with the earl appearanc~ of the ~ye, in comparison with that of other parts of th~ ?ody. It 1s greemsh at first, and passes insensibly to a deep black. fhe abdomen, after being flexed from behind forwards, bends sud~ enly backwards to form a long, slender, pointed tail, from which ~ssue two long articulated threads. The animal always swims on Its back, and most frequently in a horizontal direction by the quick and re.peated. motion of its arms and feet, and execut:s all sorts of evolutiOn~ ':ah e~se and agility. When young, and after its first cha~ge~, It 1s subJect to a disease called the ep!tippium(l ); but this e~htppiUm or saddle always has a determinate figure, and never con· tams the two oval ampullre observed in the Daphnire Th . mals do no. t live Ion g in a state of capti.V I. ty, nor can• theil·e syeo uanngt· ones be raised, at least such was the case with Jurine who could not pr~serve them after their fiJ·st changes. Among all 'th . whtch were the b · f . e specimens . 1 su JCCts o his observations, he could not find a shm g e m. ale ' th. ough ' it t· s t rue, h e cou1 d procure but very few of t em, this species being rare in the environs of Geneva. It is said, ( 1) See the following article' Daphnia, p. 128. BRANCHIOPODA. 127 however, to be very common in the marshes and ponds of the North, where it aggregates in considerable numbet·s. In the , DAPHNIA, Mull. The oars are always exposed to their base or to the origin of their eduncle; they are as long, or almost as long as the body, and are ~ivided into two branches, the posterior of which consists of four joints, the first very short, and the other, or the anterior, of three. Their eye is small or punctiform, and with the exception of certain species, has not, as in Lynceus, the small black punctiform spot before it, which Muller considered as a second eye( 1 ). Although the extreme smallness of these animals might be supposed to defy any attempt to investigate their organization, but few are better known. Exclusive of those who have devoted themselves 10 microscopic researches, four of the most profound naturalists, Sch::effer, Randohr, Straus, and Jurine, Sen., the thit·d particularly, have studied them with the most scrupulous attention. If some anatomical details escaped the notice of the latter, the omission has been remedied by the labours of Randohr and Straus; Jurine also completes the observations of the former with respect to their habits, which he studied for a long period, and with the greatest success. The mouth is situated beneath at the base of the rostrum; we consider (with Randohr) the inferior portion of the head, which Straus denominates a labrum, as an elongated clypeus, and we apply the former term to that part which he styles the posterior lobule of the labrum. Directly under it are two strong jaws-interior jaws of Randohr-without palpi, vertically inclined, and applied to two horizontal jaws(2) terminated by three stout horny spines, in the form of recurved hooks. Then come ten feet, the second joint of all of which is vesicular; the fit·st eight terminate by an expansion in the manner of a fin, the edges furnished with setce or barbed threads arranged like a crown or a comb; the two anterior seem to be specially appropriated to the purposes of prehension, and in fact Randohr considers them as double palpi, the external and internal: they are the same parts, elsewhere-Cyclops-called hands by Jurine. In the figures which they have published, the terminal setre appear to be . (1) Such also is the opinion ofUandohr, Monoc, pl. V, fig. II, iii, 6; and as he ?tscovered it in the Daphnia sima, it is possible that, although but slightly visible m several species, this character may be common to this subgenus, and that of Lynceus. Schreffer had previously noticed it. (2) The exterior jaws, in the language of Randohr; Jurine not having separated these part& from the preceding ones, supposed that the latter were accompanied by a kind of valve and by a palpus. Hist. dt!s Monoc. IX, f. lS-17. |