138 INVERTEBRATE ZOOLOGY 
the cardio-branchial to the cardio-visceral vessels, and back 
again. The contraction of the heart is of a peristaltic nature; 
it passes from one end to the other of it for a short time; then 
after a short pause the contraction is renewed, the peristaltic 
motion beginning at the opposite end and driving the blood 
in the opposite direction. 
The nervous system. About halfway between the two siphons, 
imbedded in the mantle beneath the dorsal surface of the animal, 
lies a small ganglion from which nerves radiate. No organs of 
special sense are present, except the tentacles and minute eye- 
spots at the incurrent and excurrent openings. 
The excretory system. Beneath the heart is an elongated vesic- 
ular organ which is the single, unpaired kidney; it is ductless. 
Beneath the ganglion above mentioned is a small glandular 
organ called the subneural gland; it has a duct which communi- 
cates with the pharynx. The function of this gland is probably 
excretory ; it is supposed to be homologous to the hypophysis 
of vertebrates. 
Exercise 2. Make a drawing of the left side of the animal on a 
scale of from 4 to 6, showing all the internal organs which 
appear in that aspect. Label the dorsal and the ventral 
sides of the body and all the organs. 
Exercise 3. Make a drawing of the right side of the animal 
showing all the organs which appear in that aspect. 
Bxercise 4. Make a drawing of the dorsal side showing the 
organs observed there. 
The peribranchial chamber. Cut off the excurrent siphon at its 
base and with a needle or bristle probe the opening. The probe 
will pass into the large space between the mantle and the 
pharynx. This is the peribranchial chamber; it surrounds the 
pharynx on all sides, except in the mid-ventral line, and commu- 
nicates with the outside water through the excurrent siphon. 
