20- LIVERPOOL BIOLOGICAL SOCIETY. 
of the external appearance of the animal; but it is quite 
absurd to attempt to describe or even in most cases to 
identify an Ascidian without dissection and microscopic 
examination. As Savigny long ago said,* “les Ascidies 
ont l’organisation variée et l’aspect uniforme. a con- 
figuration qui leur est affectée ne permet pas que les 
différences intérieures se manifestent au-dehors par des 
signes fort sensibles. Aussi les distinctions nécessaires 
a la parfaite connaissance des espéces sont-elle difficiles a 
tracer.” 
In some cases the genus and even sometimes the 
family cannot be determined without dissection. For 
example, in many museums and other collections, all 
Simple Ascidians which are incrusted with sand and shell 
fragments are labelled ‘‘Molgula,’ but some of these 
specimens usually belong to the genus Hugyra (to dis- 
tinguish which the branchial sac ought to be examined 
microscopically), and in some cases they belong to 
Polycarpa, a member of a different family, the Cynthiide, 
and they may even be Ascidiide (e.g. Ascidia involuta, 
Heller).. It is even possible that such forms might be 
Compound Ascidians, as Polyclinwm sabuloswm and various 
species of Psammaplidiwm are incrusted with sand, and in 
external appearance mimic the Molgulide. Consequently 
most of the older descriptions of Ascidians are of little or 
no value ; they frequently give no clue even to the genus 
to which the species belonged. 
Tt is clear then that the internal structure or anatomy 
of the animal must be considered in diagnosing the species 
of Tunicata, and the important question then arises— Which 
organs are of greatest importance in distinguishing allied 
species? All recent investigators of the Tunicata are 
agreed in fixing upon certain important organs, such as 
* Mémoires sur les Animaux sans Vertébres, Pt. ii., p. 84, 1816. 
