THE NATURAL HISTORY SURVEY. 11 
Dilg, Dr. Howard N. Lyon, Miss Grace M. Hall, Mrs. Agnes 
Chase and Mrs. Lillian M. Baker, Chicago; Mr. B. T. Gault, 
Glenn Ellyn, Ill.; Prof. Oliver Marcy, Northwestern University, 
Evanston, Ill.; Mr. Eliot Blackwelder, Morgan Park, Ill., and 
Mr. Edward H. Baker, of Providence, R. I. 
The author’s thanks are also due to Messrs. Arthur and 
John Wilkinson for the use of a valuable camera, with which 
the plates in this volume were made, and the authorities of the 
United States National Museum, who kindly opened the way 
‘for the examination of a large quantity of material from the 
collection of The Chicago Academy of Sciences. 
I. GENERAL CHARACTERS OF THE MOLLUSCA. 
It is very difficult to prepare an exact definition of the 
Mollusca. That generally given—laterally symmetrical, un- 
jointed body protected by a shell and with a creeping disk, etc. 
—is hardly correct, since in the majority of forms the lateral 
symmetry is effaced by torsion (snails) and by attached forms 
(oyster). The writer would define a mollusk as a body com- 
posed of cells which form a double sac, the outer being the 
body wall and the inner the covering of the digestive tract, the 
space between these two sacs being a ccelum or blood lymph 
space. The body is differentiated into a head (prostomium) and 
a dorsal and ventral surface and right and left sides, thus giv- . 
ing rise,in the more primitive forms, to bilateral symmetry. 
In all forms there is a creeping disk or foot, which is really the 
most characteristic part of this group. A pallium or mantle is 
always present, which secretes, in the shelled forms, the calca- 
reous exoskeleton or shell. The gills (ctenidia) are develop- 
ments from the body-wall and contain two blood vessels. The 
nervous system consists of paired ganglia or nerve masses 
which are more developed about the head than in any other 
region, and which send off other ganglia with connecting com- 
missures to other parts of the body. The circulatory system 
consists of a ventricle and one or two auricles, situated within a 
pericardium. Paired or single nephridia (renal organs) are 
present, and a more or less complicated reproductive system, 
which is in many cases hermaphroditic. In one group (Gas- 
tropoda) the mouth is provided with a manducatory apparatus 
(odontophore, radula or tooth bearer).* 
*For an excellent account of the Mollusca, see Lankester, ZoGlogical Articles, p. 95, to 
which the author would acknowledge his indebtedness for much information. 
