3°6 



MANUAL OF ZOOLOGY 



the terminal part of the intestine close to the anal aperture 

 is the duct of a peculiar gland — the ink gland (Fig. 193, 1). 

 This secretes a black substance, the ink, which is discharged 

 when the animal is irritated or alarmed, and mingling with 

 the water in the mantle-cavity is discharged as a dark cloud, 

 under cover of which the animal may elude the pursuit of 

 an enemy. 



The heart and vascular system reaches a high stage of 

 development. The heart consists of a median ventricle and 



nejbh.ap 



frtCLnt 



Fig. 191. — Nautilus pompilius, interior of mantle-cavity of a male specimen with 

 the postero-ventral wall reflected, a. 1. neph. ap, oral left nephridial aperture: 

 an, anus ; cten, ctenidia ; /. $ ap, left reproductive aperture ; /. ant. os, left oral 

 osphradium ; / vise ap, left viscero-pericardial aperture ; mant, flaps of mantle : 

 pen, penis ; /. A neph. np, aboral left nephridial aperture ; /. r. neph. tip, aboral 

 right nephridial aperture ; post, os, aboral osphradia ; ?-. vise, np, right viscero- 

 pericardial aperture. 



two or four elongated lateral auricles or branchio-cardiac 

 vessels conveying the blood from the ctenidia to the ventricle. 

 The nervous system is highly developed, and its principal 

 central parts, representing the cerebral, pedal, and visceral 

 ganglia of other molluscs, with their commissures and con- 



