OCR Text |
Show 114 CRUSTACEA. means of their feet, appear to us to establish a poRitive line of demarcation between the former and the latter. We have consulted several erudite naturalists with respect to these transformations, but none of them have observed a change of skin to occur. The antennre of the Entomostraca, whose form and number greatly vary, serve for natation in several. The eyes are J•arely placed on a pedicle, and when this is the case that pedicle is a mere lateral prolongation 'of the head, and i; never articulated at base ; they are frequently closely ap· proximated and even form but one. The organs of genera· tion are situated at the origin of the tail; it has been thought but erroneously, that their seat was in the antennre of th; male. This tai1(1) is never terminated by a fan-like fin nor does it present those false feet observed in the Malacost~ca. The ova are collected under the back, or are external, and covered by a common envelope, and resemble one or two smali clu~ters at .the base of the tail ; it appears that they can be kept m a desiccated state for a long period without losing their properties. It is only after a third change of skin that these animals become adult a~d capable of continuing their species. It has be~n proved,, With respect to some of them, that a single copu· latton fecundr.fies several successive generations. ORDER I. BRANCHIOPODA. A mouth composed of a labrum, two mandibles a ligula and one or two pairs of jaws, and branchire, the first of t. he form of the .b ody or the number of feet · The se organs m. some the Arguh. for mstance, experience changes which modif th . ' (l) If t y e1r uses. we excep the Phyllopa, the ]ast feet are thoracic, or foot-jaws (Oypril). BRANCHIOPODA. 115 h . h when there are several, are always anterior, charac-w JC ' • ten.z e thi's order or the sixth of the clas.s . These Crustacea are always wandering and are generally tected by a shell resembling that of a bivalve, and fur· p~hed with four or two antennre. Their feet, with a few rusceptions, are who11y nata tory. Their number varies, be-ex • • h . so ing but six in some, while m ot ers 1t amounts to twenty, lOr-ty ·two, or more than a hundred. Many of them have but one eye. Most of these animals, as we have already stated, being nearly microscopical, it is evident that the application of one of the characters we have employed-that of the presence or absence of the pal pi of the mandibles-with respect to them, presents almost insuperable difficulties(!). The form and number of the feet, that of the eyes, the shell, the antenl).re, furnish us with more visible marks, and such as are within the observation of every one. This order in the systems of De Geer, Fabricius and Linnreus, a single species excepted-M. polyphemus, contained but the single genus MoNocuLus, Lin. (2) Which w~ will divide into two principal sections. The first,-that of the LoPHYROPA-is distinguished by the number of feet, which never extends beyond ten; their joints are also more or less cylindrical or conical, and never eu tirely lamelliform or foliaceous; the branchire are but few in number, and most of them have but one eye. Several, besides, have mandibles provide.d with a palpus(3); there are, almost always, four antennre which serve for locomotion. In the second section-that of the PHYLLOPA-the number of feet is increased to at least twenty, and in some amounts to many more; their joints, or at least the last ones, are flattened and resemble cili- (1) We will begin, however, with those llranchiopoda whose mandibles ~e furnished with palpi; they constitute the two first divisions of the Lophyropa. (2) And that of Binocle in the system of Geofii·oi. (3) M. Straus appears to attribute t~is character exclusively to Cypris and Cytherea, which compose his order of the Ostrapoda; but from the observations of Jurine, Sen., and Handohr, it seems that it also belongs to Cyclops. |