New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

MALACOSTRACA
“Challenger" Report on Schizopoda, by Sars, 1885, dealing with
the order at large ; “ British Schizopoda,” by Norman, Ann. Nat.
Hist., 1892; “ Decapoden und Schizopoden,” Plankton - Expedi¬
tion, Ortmann, 1893 ; “ Euphausiidae, ” by Stebbing, Proc. Zool.
Soc., London, 1900; Mysidoe of the Russian Empire, by Czerniavski,
1882-83 ; and Mysidoe of the Caspian, by Sars, 1893-95-97.
4. Stomatopoda.—This order, at one time a medley of hetero¬
geneous forms, is now confined to the singularly compact group
of the Squillidse. Here the articulation of the ocular segment is
unusually distinct, and here two characters quite foreign to all the
preceding groups come into view. The second maxillipeds are
developed into powerful prehensile ox-gans, and the branchiae, in¬
stead of being connected with the appendages of head and trunk,
are developed on the pleopods, appendages of the abdomen. At
least three segments of the trunk are left uncovered by the cara¬
pace. The developing eggs are not carried about by the mother,
but deposited in her subaqueous burrow, “where they are
aerated by the currents of water produced by the abdominal feet of
the parent.” An excellent synopsis of the genera and species is
provided by R. P. Bigelow {Proc. U.S. Mus. vol. xvii., 1894). For
the habits and peculiarities of these and many other Crustaceans,
Verrill and Smith on the Invertebrates of Vineyard Sound should
be consulted (1874). The general subject has been illuminated by
the labours of Claus, Miers, Brooks (“ Challenger ” Report, 1886),
and the latest word on the relationship between the various
larvae and their respective genera has been spoken by H. J.
Hansen {Plankton-Expedition Report, 1895), The striking forms
of Alima and Erichthus, at one time regai’ded as distinct genera,
ai’e now with more or less certainty affiliated to their several
squillid parents.
5. Sympoda..—This order of sessile-eyed decapods was abso¬
lutely unknown to science till 1779. A species certainly belong¬
ing to it was described by Lepekhin in 1780, but the obscure
Gammarus esca, “ food Gammarus ” beloved of herrings, de¬
scribed by J. C. Fabricius in the preceding yeax-, may also be one
of its members. Nutritious possibilities are implied in Diastylis
rathkii (Kroyer), one of the lai’gest forms, which, though slender
and rarely an inch long, in its favourite Arctic waters is found
“in incalculable masses, in thousands of specimens ” (Stuxberg,
1880). Far on in the 19th century eminent naturalists were still
debating whether in this group there were eyes or no eyes,
whether the eyes were stalked or sessile, whether the animals
observed were larval or adult. The American Say in 1818 gave a
good description of a new species and founded the premier genus
Diastylis, but other investigators derived little credit from the
subject till more than sixty years after its introduction by the
Russian Lepekhin. Then Goodsir, Kroyer, Lilljeborg, Spence
Bate, and one or two others made considerable advances, and in
1865 a memorable paper by G. 0. Sars led the way to the great
series of reseai’ches which he has continued to the present day.
The name Cumacea, however, which he uses cannot be retained,
being founded on the preoccupied name Cuma (Milne-Edwards,
1828). The more recent name Sympoda (see Willey, Results, part
v. p. 609, 1900) alludes to the huddling together of the legs,
which is conspicuous in most of the species. Ten families are
now distinguished — Diastylidaee, Lampropidse, Platyaspidse,
Pseudocumidie, all with an articulated telson ; without one, the
Bodotriidae (formerly called Cumidce), Yaunthompsoniidse, Leu-
conidse, Nannastacidse, Gampylaspidae, Procampylaspidse. All the
Leuconidae and Procampylaspidae are blind, and some species in
most of the other families. Usually the sides of the carapace are
strangely produced into a mock rostrum in front of the ocular
lobe, be it oculiferous or not. The last four or five segments of
the trunk are free from the carapace. The slender pleon has
always six distinct segments, the sixth carrying two-branched
uropods, the preceding five armed with no pleopods in the female,
whereas in the male the number of pairs varies from five to none.
The resemblance of these creatures to miniature Macrura is alluded
to in the generic name Nannastacus, meaning dwarf-lobster. In
this genus alone of the known Sympoda the eyes sometimes form
a pair, in accordance with the custom of all other malacostracan
orders except this and of this order itself in the embryo (Sars,
1900). The most but not the only remarkable character lies in the
first maxillipeds. These, with the main stem more or less pedi-
foi'm, have the epipod and exopod modified for respiratory pur¬
poses. The backward-directed epipods usually carry branchial
vesicles. The forward-directed exopods either act as valves or
foi'm a tube (rarely two tubes), protensile and retractile, for regu¬
lating egress of water from the branchial regions. This mechan¬
ism as a whole is unique, although, as Sars observes, the epipod
of the first maxillipeds has a respiratory function also in the
Lophogastridae and Mysidae and in the cheliferous isopods. As a
rule armature of the carapace is much more developed in the com
paratively sedentary female than in the usxxally more active male.
Only in the male do the second antennae attain considerable length,
with strong resemblance to what is found in some of the Amphi-
poda. About 150 species distributed among thirty-four genera
481
are now known, many from shallow water and from between tide-
marks, some from very great depths. H. J. Hansen concludes
that “they are all typically gi’ound animals, and as yet no
species has been taken under such conditions that it could be
reckoned to the pelagic plankton. ” As they have been found in
all zones and chiefly by a very few observers, it is probable that
a great many more species remain to be discovered. In recent
years thirteen species, all belonging to the same genus Pseudo-
cuma (Fig. 3), have been recorded by Sars from the Caspian Sea. A
Fig. 3.—Pseudocuma pectinatum, Sowinsky.
bibliography of the order is given in that author’s Crustacea
of Norway, vol. iii., 1899-1900.
6. Isopoda.—This vast and populous order can be traced far
back in geological time. It is now represented in all seas and
lands, in fresh-water lakes and streams, and even in warm springs.
It adapts itself to parasitic life not only in fishes, but in its own
class Crustacea, and that in species of every order, its own in¬
cluded. In this process changes of structure are apt to occur,
and sometimes unimaginable sacrifices of the normal appearance.
The order has been divided into seven tribes, of which a fuller
summary than can here be given will be found in Stebbing,
History of Crustacea, 1893. The first tribe, called Chelifera,
from the usually chelate or claw-bearing first limbs, may be re¬
garded as Isopoda anomala, of which some authors would form a
separate order, Tanaidea. Like the genuine isopods, they have
seven pairs of trunk-legs, but instead of having seven segments
of the middle body (or perseon) normally free, they have the first
one or two of its segments coalesced with the head. Instead of
the breathing organs being furnished by the appendages of the
pleon with the heart in their vicinity, the respiration is controlled
by the maxillipeds, with the heart in the perseon (see Delage,
Arch. Zool. exper. et gin. vol. ix., 1881). There are two families,
Tanaidse and Apseudidae. Occasionally the ocular lobes are
articulated.
The genuine Isopoda are divided among the Flabellifera, in
which the terminal segment and uropods form a flabellum or
swimming fan ; the Epicaridea, parasitic on Crustaceans ; the
Valvifera, in which the uropods fold valve-like over the branchial
pleopods ; the Asellota, in which the first pair of pleopods of the
female are usually ti’ansformed into a single opercular plate ; the
Phreatoicidea, a fresh-water tribe, known as yet only from sub¬
terranean waters in New Zealand and an Australian swamp nearly
6000 feet above sea-level; and lastly, the Oniscidea, which are
tei’restrial. Only the last of these, under the contemptuous desig¬
nation of wood-lice, has established a feeble claim to popular
recognition. Few persons hear without surprise that England
itself possesses more than a score of species in this air-breathing
tribe. Those known from the world at large number hundreds
of species, distributed among dozens of genei’a in six families.
That a wood-louse and a land-crab are alike Malacostracans, and
that they have by different paths alike become adapted to tei’res¬
trial life, are facts which even a philosopher might condescend to
notice. Of the other tribes which are aquatic there is not space
to give even the barest outline. Their swarming multitudes are
of enormous importance in the economy of the sea. If in their
relation to fish it must be admitted that many of them plague the
living and devour the dead, in return the fish feed rapaciously
upon them. Among the most curious of recent discoveries is that
relating to some of the parasitic Cymothoidce, as to which Bullar
has shown that the same individual can be developed fii’st as a
male and then as a female. Of lately-discovered species the most
striking is one of the deep-sea Cirolanidae, Bathynomus giganteus,
A. M. Edwards, 1879, which is unique in having supplementary
ramified branchiae developed at the bases of the pleopods. Its eyes
are said to contain nearly 4000 facets. The animal attains what
in this order is the monstrous size of 9 inches by 4. A general
uniformity of the trunk-limbs in Isopoda justifies the ordinal
name, but the valviferous Astacillidae, and among the Asellota
the Munnopsidae, offer some remarkable exceptions to this charac¬
teristic. Among many essential works on this group may be
named the Monogr. Cymothoarum, of Schiodte and Meinert, 1879-
1883 ; “Challenger" Report, Beddard, 1884-86 ; Cirolanidae, H. J.
Hansen, 1890; Isopoda Terrestria, Budde-Lund, 1885; Bopy-
ridce, Bonnier, 1900; Crustacea of Norway, vol. ii. (Isopoda),
Sars, 1896-99, while their multitude precludes specification of
important contributions by Benedict, Bovallius, Chilton, Dohrn,
Dollfus, Fraisse, Giard and Bonnier, Harger, Haswell, Kossmann
s. VI. — 61