380 GEOLOGICAL SURVEY OF THE TERRITORIES. 
in Apus should be regarded as the first abdominal pair or not must, it 
seems to us, remain an open question; there seems, however, to be no — 
other line of demarkation in the family which this genus represents. 
The telson.—This portion of the abdomen, sometimes called “post- 
abdomen,” is large and well differentiated in the two lower families, 
especially i in Estheria—where it 1s compressed, high, armed above with 
numerous spines, and bearing below a pair of “nodified caudal ap 
pendages which we shall consider under the head of the appendages. 
We will rapidly recall the salient points in the fourm of the telson in 
the three families of Phyllopods. In Limnetis the telson is much as in 
some Cladocera, being small, without teeth along the upper edge, though 
still bearing the pair of dorsal filaments (see fer, 4, in text, and Plate 
I, fig. 6). These are also present in certain Oladocera, Daphnia, Bosmina, 
&e., and in several genera allied to Daphnia, Alona, ‘Pleurowus, &e., the 
upper edge is dentate or spined; this with other features in Limnetis 
shows that the Phylopods have probably descended from Cladocera-like 
ancestors. In Hstheria and Limnadia the telson is large and densely 
spined along the upper edge. The large spivy telson is probably of 
use to aid the animal in pushing itself through submerged dense vege- 
tation, for all that portion of the body which can be thr ust out between 
the valves is armed with stout spines, whereas in Limnetis, only the 
telson can project beyond the edge of the carapace valves. 
In Apus the telson is nearly cylindrical, short and small, and flattened 
from above downwards, and is without much functional value, though 
the cercopods are of use in swimming; but in Lepidwrus it is produced 
into a long spatulate portion like a beav er’s tail, and which must give 
it an advantage over Apus in extricating itself from muddy places. 
In the Branchipodide the telson assumes the form of a simple segment, 
cylindrical, soft, unarmed, but in Thamnocephalus becoming very broad 
and flattened into a lateral fin-like expansion and without any caudal 
appendages, which are always present in the other genera of the family. 
What we call the telson, and which is simply the last abdominal seg- 
ment, is called by Gerstaecker, and we suppose earlier authors, the “post: 
abdomen.” Some authors, the most recent, Gerstaecker in Bronn’s 
Classen und Ordnungen, &e., speak of the “ abdomen ” and “ postabdo- 
men” in the Br anchipodida, but they do not state where the abdomen 
begins. The term “ postabdomen” is applied to the last eight (Artemia) 
or nine (Branchipus, &c.) segments of the body (uromeres), but we see 
no good reason for not regarding these segments as forming a true ab- 
domen (urosome), the first segment, or ninth from the end, in Branchi- 
pus bearing the external reproductive organs. We really see no need 
of employing the term “postabdomen” in speaking of any Branchipod, 
nor in fact, so far as we are aware, does it have any special significance 
in other groups. We here consider the so-called ‘“‘postabdomen” of the 
Limnadiade and Apodide as the telson, and the homologue of the telson 
in the macrurous Decapoda. 
The eyes.—There are in the lower Phyllopoda but a single pair of com- 
pound or facetted eyes, but in the Branchipodide the simple, unpaired 
eye of the larva is retained. In the Limnadiade these are sessile; in 
the Branchipodide they are stalked. The structure of the eye of Apus 
cancriformis and Branchipus (species not named) has been described 
and in part figured by Grenacher in his great work “‘ Untersuchungen 
iiber das Sehorgan der Arthropoden” (1879). The eye of Branchipus 
_ Stagnalis has previously been investigated by Leydig in 1851; and that 
of Hstheria californica by Lenz in 1876. The eye of Limnetis has appar- 
ently not been investigated, and our own observations on it are but 
