Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/684

650 CRUSTACEA Trilobiten stadium of Dohrn 1 resembles Prestwichia rotundata from the Coal-measures far more than any known Trilobite. Packard has figured an earlier (1) stage of Limulus than Dohrn s " Trilobiten-stadium," which much more nearly resembles Trimicleus^ (compare figs. 51 and 55). In its earlier stages the young Limulus can roll its posterior segments under its head-shield, and when it at last leaves the egg it can swim well, and has been captured by Alexander Agassiz, swimming freely on the surface of the ocean, three miles from Naushon Island, Buzzard s Bay. 3 At this period it has no caudal spine or telson ; this is acquired only at a later moult, whilst a year or more elapses before the young males can be distinguished by their modified antennas from the females (see 3a in fig. 12). In the shield-bearing, naked, and bivalved Phyllopoda, represented by Apus, Nebalia, Branchipiis, and Estheria, 2> L S FIG. 56. Estheria, sp. D, from Dubuque, Iowa; () the eye. L, from Lynn, Massachusetts (nat, size). <S presents a highly magnified section of one of the valves to show the successive moults. B, an enlarged portion of the edge of the shell along the back, showing the overlap of each growth. (Morse s Zoology). the embryological development is no doubt analogous, though less is known in regard to Estheria (fig. 56). In Apus the male is not certainly known, all the progeny observed being fertile females. Probably, however (as is the case in Daphnia), males appear at a particular season of the year, and these suffice to render fertile several gener ations of females. The females let their eggs fall to the bottom of the water, where they remain until hatched by the sun s warmth. The eggs of Apus occupy from two to there weeks in hatching ; the chorion splits, as in Limulus, revealing a semi-transparent inner egg-membrane. This also soon after bursts, giving freedom to a simple nauplius like that exceptionally met with by Fritz Miiller in a prawn allied to Pencens. The large cephalic shield, so character istic of the adult Apus, is not seen in these early stages, and when it first appears, it closely resembles that of Pellocaris, an extinct Silurian form (4 in fig. 57). The nauplius, being short -bodied, does not display those graceful undulatory motions in the water observed in the adult, but progresses rather by a series of jerks like the adult Cyclops. At the end of eight or ten days the young Phyllopod has acquired considerable size. The body-seg- 1 Dohrn, op. cit. p. 639, Taf. xv. fig. 4. a Salter supposes (Quart. Journ. Geol. Soc., vol. iii., 1847, p. 252) that the membranous margin of the head-shield of Trinucleus was once entire, " then became plicate, then perforate, and lastly separated into linear processes." It seems more probable that the margin of the head-shield was originally digitate, then gradually closed up, leaving only perforations along the sutures in some, and only plicoe in others. We have an analogous case in Ifaliotis, Scissurella, and Pleuroto- maria amongst the Mollusca, in which a slit becomes partially or wholly closed up, leaving perforations at intervals. In the Mollusca it is connected with the respiratory functions, but in Limulus, Hernias- pis, and Trinucleus it is probably a remnant of the margins of the primitive segments which have coalesced to form, the cephalic shield. 3 Packard, op. cit. p. 155. ments and feet, so numerous in Apus, Artemia, and BrancJiipus (5a in fig. 57), are formed gradually (in repeated moults) from before backwards, without any sharply-defined regions of the body being discernible either by the time of FIG. 57. PHYLLOPODA. 1, Ceratiocaris papilio, U. Siluriun, Lanark; 2, bipes(one side of carapace removed to show branchial feel), Marine British ; 3, Lepidurui Angassi : a, dorsal aspect; 6, ventral aspect of head showing the hypostome and mandibles ; hab. freshwater, Australia; 4, larva of Apuscancri- formis; 5, Branchipus stagnaUs : a, adult female ; 6, first larval stage ; c, second larval stage; 6, larva of Artemia satinet. their appearance or their form. All the feet are of the same pattern, and resemble the maxillae of the higher Crustacea. 4 The young animal exuviates about twenty times during the first two or three months ; it is then full grown, and in every respect resembles the parent. Nebalia (2 in fig. 57) presents a remarkable exception to the rest of its order, the young apparently (like Daphnia) undergoing no metamorphosis after they quit the egg. Metschnikoff, who has recently studied the development of Nebalia, states that he has observed that it passes through both a nauplius and a zoe a stage within the egg, 5 and he therefore regards Nebalia as a Phyllc pcdiform Decapod. We hardly see sufficient grounds at present for reigning Nebalia to a higher order than that in which it is r.cw placed. The Cladocera do not afford cny additional aid in embryological research. They appear to quit the egg c nly smaller than the parent, but with their full number of limbs. Of the developmental history of the Ostracoda but little is known. Zenker states that their anterior lin:bs are developed first, and the youngest stages, according to Clans, are shell-bearing nauplius-forms. In theCopepoda (3-7 in fig. 58), the buckler does not cover more than the head and thorax, the abdominal segments, which are nearly cylindrical, extending beyond it. They are met with both in freth waters and in the sea all over the world, and are most numerously represented both in a free state and as parasites. The larvae of the non-parasitic forms (Bb-d in fig. 58), all possess at the earliest period the three anterior pairs of limbs, i.e., the future antennae and 4 " The maxilla of the Decapod larva is a sort of Phyllopodal foot" Clans). "We might," says Fritz Miiller, "regard the Phyllopoda as zoeae which have not arrived at the formation of a specialized thorax or abdomen, but have instead repeatedly reproduced the appendages which first follow the nauplius limb." The present writer has com pared the Decapod larva in which the maxillce serve temporarily as organs of natation and locomotion with the similar appendages which persistently fulfil this office in Pterygotus, Stylomirus, and Limulus (H. Woodward, Mon. Pal. & oc. Mcrobtomata). 6 The greatest caution should be exercised in instituting comparisons between the so-called "nauplius" and "zoe a " stages of any one Crus tacean, when such stages are passed within the egg, and those of any other Crustacean whose young actually pass through such stages after they have quitted the egg. In the Decapoda we at present know of only one instance in which the young appears as a free-swimming navplius; in the majority we see only the zoeal and larval stages ; in some even the zoeal stage is overleaped, and the young appears as a larva differing

but little, if at all, from the parent.