THE HETEROPOD MOLLUSKS. 251 
the tongue or radula has highly characteristic teeth, which 
serve these rapacious animals to seize their prey. The in- 
testine runs straight back from the mouth, and after mak- 
ing one or two coils ends in the vent. The excretory organs 
open near the anus; the contractile tube opens internally 
into the pericardial cavity, and resembles in form and posi- 
tion the excretory organ of the Pteropoda. The circula- 
tion is imperfect, the blood passing from the wide sinuses 
of the body to the ventricle of the heart. From the auricle 
springs the aorta, which subdivides into several branches 
that open freely into the body-cavity. The circulation can be 
easily watched, owing to the transparency of the body. The 
aération of the blood is effected partly through the skin, 
partly through gills, except in a few species. The branchize 
are either thread- or leaf-like ciliated appendages, which 
may either be free or enclosed in the mantle-cavity. The 
sexes are distinct. The males can be readily recognized by 
the large copulatory organ, which hangs free on the right 
side of the body. The sexual glands fill the posterior por- 
tion of the visceral cavity, and are partly imbedded in the 
liver. The oviduct is complicated by the presence of an 
albumen gland and a receptaculum seminis. It opens on 
the right side of the body. 
The Heteropods are exclusively marine, but are found in 
all quarters of the world. The number of species is small, 
and there are two orders only—the Plerotracheide with a 
small or no shell and free gills, and the Atlantide with a 
large coiled shell and gills placed in the mantle. Ptero- 
trachea (Firola) coronata Forsk. is found in the Mediter- 
ranean, and on account of its transparency has often been 
investigated. The Heteropoda live together in large num- 
bers, and feed on small animals. 
The eggs are laid in cylindrical strings, which soon break 
up into numerous pieces. The segmentation of the yolk is 
complete but irregular. The embryo rotates within the egg 
during the veliger stage, when it has two distinct sails, or 
lobes of the velum, and a ciliated foot with an operculum. 
In this form it leaves the egg. The velum enlarges and 
forms several divisions. The otocysts, eyes, and tentacles are 
