140 
MICROSCOPICAL SECTION. 
February 20th, 1865. 
Dr. W. C. Williamson, F.R.S., in the Chair. 
Dr. Alcock showed mounted specimens of the Carapaces 
of Entomostraca, picked from shore sand from the coast of 
Galway. They included Cythere albo-maculata, angustata, 
variabilis, flavida, convexa, impressa, pellucida, and quadri- 
dentata ; besides about thirty other species of Cythere and 
Cythereis which are not described in Dr. Baird’s Monograph. 
Professor Williamson said that the further we extend 
our observations of the lower forms of animal life, the greater 
become our difficulties in determining specific distinctions ; 
and in the present case it must be remembered that we have 
only the shell or carapace for examination, and this outer 
skin is of less value for distinctive characters than the internal 
parts. Then again it is known that some of the entomostraca 
undergo several metamorphoses similar to those passed 
through by the higher Crustacea before they become adult, 
so that he should not be at all surprised if some ten or 
twenty of these different forms turned out to belong to one 
and the same species. There still remained a third difficulty, 
the greatest of all, namely, the doubtfulness of what a species 
is ; and in the lower forms of life the variability is so great that 
he doubted if specific distinctions could be made with any 
certainty. The same remark is applicable to the lowest 
forms of the higher divisions of animals, and the entomostraca 
standing low in the articulate class are probably subject to 
similar variability. 
