33 GEOLOGICAL BIOLOGY. 



very thick, enrolled muscular fold, of which the external borders are in. 

 terlaced one with the other; the tentacles and funnel, as the innervation 

 shows, correspond to the foot of the Gastropod. At the base of the lateral 

 ocular tentacles is found on each side a large eye, with short peduncle; 

 in the midst of the crown of tentacles is situated the buccal (mouth) cavity 

 surrounded by thick walls, with a fleshy tongue, the root of which is com- 

 posed of many series of plates and hooks. The jaws, of extraordinary 

 strength, recall in form the beak of a parrot. The large branchias, or 

 gills, are found in two pairs at the base of the funnel: they penetrate 

 freely into the respiratory cavity; between these open the anal orifice 

 and a little further back, the organs of generation. 



The respiratory cavity and head are covered by a thin lobe or mantle, 

 -especially developed on the ventral side, and secreting the shell of the 

 outer chamber. 



The animal is attached to the shell by a powerful muscle of oval form, 

 placed below the eyes, and inserted on the internal wall of the chamber 



FIG. 102. Nautilus pompilius. (After Owen.) a mantle ; b = dorsal aspect of mantle ; c = 

 hood ; d funnel ; e = nidamental gland ; h shell muscle ; o = eye. 



of habitation, where it leaves slight impressions. From the rounded 

 posterior extremity of the animal proceeds a membranous hollow cord, 

 furnished with blood-vessels, the Siphon, which passes by a rounded 

 opening through the last partition-wall into the chambered part of the 

 shell, and continues thus in an uninterrupted manner to the initial chamber. 

 The Shell. By the internal chambering or partitioning of the shell 

 (Fig. 102), which is characteristic of them, the shells of the Tetrabranchiata 

 are distinguished from all the shells of Mollusca hitherto considered. The 

 last, distinguished by its greater capacity, serves as the chamber of habi- 

 tation for the animal; all the rest of the shell is divided into chambers by 

 transverse partitions, called Septa, which succeed each other at regular 

 intervals. The chambers are filled with air (gas), and united together by 

 the Siphon. The exterior form of the shell presents extraordinary varia- 

 tion; in general, it may be considered as a straight conical tube, aug- 

 menting little by little in thickness, which continues to incurve, sometimes 

 in a straight line, and often in a curved line. There are, consequently, 



