B ranchiopoda. 
27 
pools and ditches in many parts of Europe, but it is very uncertain 
in its occurrence, and it may suddenly reappear in numbers after 
an absence of many years. Males are rarely found. It was 
formerly found in several localities in the South of England, but 
no British specimens were seen for upwards of forty years, and 
the species was supposed to be extinct in this country. In 1907, 
however, it was discovered by Mr. E. Balfour Browne, in Kirk- 
cudbrightshire, and some specimens obtained by him are exhibited. 
The eggs of Apus, and indeed of most Branchiopoda, can survive 
being dried, and they may be carried from place to place in mud 
adhering to the feet of wading birds 
or in other ways. There can 
be little doubt that the recent 
appearance of the species in Scot- 
land was due to introduction of 
the eggs in some such way from 
the Continent. 
The species of the Sub-order 
Conchostkaca have the body 
enclosed in a bivalved shell, which 
resembles very closely the shells 
of some Molluscs. The genus 
Estheria (Fig. 8), of which speci- 
mens are exhibited, is of interest 
on account of its geological antiquity ; fossils referred to the 
genus occur in rocks of the Devonian period. 
Fig. 8. 
Estheria melitensis (slightly en- 
larged). [Table-case No. 1.] 
Order 2. — Cladocera. 
The number of somites is small. There are from four to six 
pairs of trunk-limbs. The carapace generally forms a bivalve 
shell, enclosing the body and limbs but leaving the head free. 
The antennae are large and two-branched, and are used in 
swimming. 
The Cladocera are generally very small animals, and from their 
jerky mode of swimming have received the name of “Water-fleas.” 
They are abundant everywhere in ponds and ditches, and a few 
species are found in the sea. 
One of the commonest species in fresh water is Daphnia pulex, 
of which specimens are exhibited together with an enlarged draw- 
ing of the animal as seen under a low power of the microscope 
Table-case 
No. 1. 
