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. 



Exercise 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. 



