Internally the drill may be divided into two major body spaces which 

 house the principal organ systems. The anterior space, or cephalic cavity, 

 rests upon the heavy musculature of the foot, and contains the inverted resting 

 proboscis, the central nerve ganglia, two pairs of salivary* glands, the gland of 

 Leiblein of unknown function, the esophagus with its attendant pharynx of Leib- 

 lein, and the large cephalic aorta. The posterior space, or visceral cavity, is 

 located within the shell and contains the stomach, intestine,, digestive glands, 

 reproductive organs, heart, and kidney. According to tests by Sizer (1936) the 

 flesh of drills maintains a salinity of 2 . 42 o/oo 



Nervous System 



The central nervous system of the drill consists of a number of .ganglia 

 which are eon&entrated in the head region in a mass the shape of a doughnut 

 surrounding the esophagus and the large cephalic aorta. From this center large 

 nerves radiate to all the principal organs of the body . The tentaeles ; front portions 

 of the head and foot, siphon, osphradium, gill, and proboscis are especially 

 heavily innervated The proboscis alone possesses 7 pairs of distinct nerves 

 which ramify into the principal structures of this active'"organ . 



Urosalpinx possesses sensory organs which respond to at least four dif- 

 ferent kinds of stimulation: touch, sediment concentration, light., and smell -taste 

 (chemical) . Tactile orgaos are apparently present over the entire exterior surface 

 of the soft parts of the drill . Although no experiments are reported, it may be 

 assumed that the eye spots qn the tentacles play a part in the response of the drill 

 to light. Most external surfaces appear to be sensitive to strong chemical stimula- 

 tion , Relatively dilute extracts of food (living or dead) may be detected principally 

 by the anterior portions of the head and siphon . The proboscis probably plays but 

 a small part, if any, in the initial location of prey, as it is lodged within the 

 cephalic cavity and has no direct contact with the exterior when the false mouth is 

 closed. The fully innervated tip of the everted proboscis is used in locat ing fofsiL 

 close at hand and in selecting drilling points on the shell of its prey 



Circulatory System 



The heart consists of a thin walled auricle and a strong muscular ventricle . 

 A cephalic and a visceral aorta spring from the ventricle, the former to pass into 

 the cephalic cavity and the latter to the organs in the visceral hump within the 

 shell. The cephalic aorta runs forward through the central nervous system and 

 there branches, sending one branch into the musculature of the foot and the other 

 into the proboscis . These arteries play an important role in the movement and 

 feeding of the drill-. The -artery passing into the foot transports blood which 



20 



