928 The American Naturalist. [November, 
was fully described in my paper, Parker thinks he has posi- 
tive evidence that it is a spleen, and hence uses this designa- 
tion throughout. The name was first applied to this structure 
by Klein. The term spleen in anatomy is used to designate a 
definite body of lymphoid tissue which is usually more or less 
closely connected with some part of the mid-gut, though it 
may lie in the mesentery far from the walls of the gut. The 
name is strictly morphological in its}bearing, and does not 
carry with it the idea of specific and localized functions. So 
far as the mass forms a discrete body, the term spleen is appro- 
priately applied to it, for it serves to definitely mark the mass, 
but when the mass is absent, or, in other words, when the tis- 
sue has other relations, e. g., as in the Dipnoi where it is in- 
closed wiTHIN the wall of the gut, we not only do not gain in 
the accuracy of designation, but we detract from the definite- 
ness of the name as applied to other forms. 
In structure, these tissues are not to be distinguished from 
each other, and if the aggregation in theregion of the mid-gut 
is to be called spleen, those in the hind-gut and fore-gut are 
likewise spleens. We avoid difficulties of nomenclature if we 
reserve the term spleen for a discrete mass of lymphoid tissue 
which lies in the mesentery outside of the walls of the mid- 
t. 
Professor Parker has done a great service in tracing out the 
extent of the pancreas which, in the Dipnoi, lies entirely 
within the walls of the gut between its two coats and which 
he has described in histological detail for the first time; but 
the discovery of the pancreas and its ducts was made by Mc- 
Donnell in 1858. This investigator made observations on 
living material which was afterward used for dissections, and 
he clearly states that the pancreatic duets empty into the mid- 
gut in company with the bile ducts. Melvilleadded a note to 
McDonnell’s paper to the effect that both spleen and pancreas 
were present in the organ which the latter called pancreas. 
No one has yet pointed out the very great significance which 
the condition ofthe pancreas in Lepidosiren has from a compara- 
tive anatomical standpoint. It is by far the most primitive 
condition of the organ known for the VERTEBRATA, since it 
