48 
Order I. 
The legs are provided with leaf-like gills, which 
serve for respiration. The eyes are sometimes simple 
and sometimes compound eye-points. There are 
great differences in other respects in their form and 
structure, especially as one subdivision is parasitic, 
although most of the species of the order swim freely 
in the water. 
Suborder I. Phyllopoda. 
In the Fhyllopoda the body is distinctly seg- 
mented, Among the interesting species which it in- 
cludes, is Artemza salina, the Brine Shrimp (fig. t). 
It is a small long shell-less creature with 11 pairs 
of leaf-like gill-feet, and a many-jointed abdomen, 
and is not found only in the sea,’ but also in arti- 
ficial salt-pits, and can adapt itself to the amount of 
salt in its surroundings to an amazing existent. 
Another species of this group, Apus cancriformis 
(fig. u) is about 2 inches long, and has a flat curved shell, 
40 pairs of legs, and two long terminal threads on 
the abdomen. It lives in rivers and streams, and 
sometimes appears in great numbers after violent 
storms. When its residence is dried up, it disappears 
sometimes for years, and reappears quite suddenly. 
Brancniopoda. 
This is owing to the fact that its eggs are laid in 
the mud, and retain their vitality for many months. 
Suborder II. Cladocera. 
The few minute animals classed here have an 
unjointed body, generally enclosed in a bivalve shell, 
and their dancing movements are caused by large 
oar-like organs, as well as by their swimming-legs. 
These organs consist of the modified outer antenne. 
Fig. v. Daphuia pulex, the Common Water- 
flea, lives in standing water in such swarms in spring 
and summer as often to colour it red. In winter, 
the animals die, leaving their eggs behind them. 
One very long species is Leptodora hyalina, which 
is completely transparent, and invisible in water. 
Suborder IH. Branehiura. 
A few fresh-water species are included in this 
division, one of which, Argulus foliaceus, the Carp 
Louse, is represented at fig. w. The four long cleft 
legs and large eyes show that this animal is not a 
permanent parasite, and it is now known to stray 
from one fish to another, where it fixes itself by 
its tube-like mouth, which is set with sharp teeth. 

Order II. 
The body of these small Crustacea is entirely 
enclosed in a bivalve shell. The antennze and the 
hinder pair of legs serve as swimming organs, and 
the animals dart about in the water with great 
Order III. 
This Order is divided into two sectlons, which 
it will be well to discuss separately. 
The free-swimming Copepoda natantia may be 
easily studied in the genus Cyclops, which is a com- 
mon inhabitant of fresh water. They may be dis- 
tinguished from the Ostracoda, in whose company 
they are met with, by the jointed shell-less body, 
and the females may be known by the two egg-sacs 
which are appended to the abdomen. These pretty 
little animals are an important addition to the food 
of fish; and their marine allies, Cefochi/us, are met 
with in vast multitudes, and form the chief food of 
the whale. 
The creatures belonging to the second sub- 
order, Copepoda parasita, are very different indeed 
in appearance. Ugly as they are, they are interest- 
ing as furnishing a striking example of the adaptation 
Order IV. 
Ostracoda. 
| agility. Species of the genus Cypris (C. fuscus is 
represented on fig. x) are nearly always to be found 
if looked for in summer. They are about the size 
of a pin’s head. Other genera are found in the sea. 

Copepoda. 
and retrogression of disused organs. Even these 
parasites quit the egg as a Nauplius with an eye 
and limbs, and swim about, but as soon as the fe- 
male (which alone is parasitic) finds her way into 
the gills or other organs of a fish, she grows rapidly, 
and loses all her limbs. The eye and limbs have 
become useless, and retrograde; the body loses its 
segmentation, becomes bag-like, and acquires all 
kinds of strange-looking excrescences. While the 
parasitic females grow large, and sink to a lower 
level in the animal kingdom, the males remain small, 
but retain their segmentation, limbs, and organs of 
sense, and attach themselves parasitically to the body 
of the female. In the Lerne@id@, the female, even 
before the ovaries are developed, surpasses the male 
3000 times in bulk. One of these strange female 
forms, Lerne@a branchialis, is represented at fig. y. 

Cirripedia. 
Plate XXIII (left side> 
Thoraciea. 
Although most of the species belonging to this 
section are not parasitic, yet they exhibits little out- 
ward resemblance to the more typical Crustacea. 
Here also the young exhibit the true NVauplius-form, 
but the developed animals fix themselves to various 
Suborder J, 
| objects by the head, and become surrounded by a 
shell consisting of one or several pieces, The body 
is not jointed, and the limbs are modified into 
branching organs to sweep the water into currents, 
and thus to draw food between the shells... These 
animals are hermaphrodite. They are called Barnacles. 
Fig. p. Lepas anatifera is to be found every- 
