METAZOA MOLLUSCA. 249 



be observed that the septa are much nearer together in 

 the adult than in the young (PI. 595, fig. i). If a sec- 

 tion of an old shell is examined the septa are sometimes 

 found even nearer together than in the ephebic stage. 

 This is especially interesting and exceptional, since just 

 the opposite condition of things exists in the old age or 

 gerontic stage from what has been described in the young 

 stage. 



A study of the young and the adult shell leads to a 

 study of the fleshy parts of the animal. No. 597 is an 

 alcoholic specimen of the Nautilus. It is placed in its 

 natural position with the ventral side below. The large 

 head and body of the animal occupy the outer chamber 

 of the shell, and the long fleshy tube or siphon extends 

 from the posterior part of the body backward to the inner- 

 most chamber. It is supposed by some that the cham- 

 bers of the shell are supplied with gas and by others with 

 liquid. On the ventral side the two flaps of the mantle 

 have united, forming an ambulatory pipe or hyponome 

 by means of which the animal is propelled through the 

 water. The motions of this organ have caused a sinus 

 in the aperture of the shell (see Nos. 592-594, 596), and 

 the additional layers made to the shell, indicated by the 

 lines of growth, bend backward running parallel with this 

 sinus. As we have already said, no sinus occurs in the 

 young Nautilus, so that probably at this stage it is not 

 a swimming animal. The mouth of the creature is sur- 

 rounded by arms for obtaining food, but these are with- 

 out sucking cups. 



The edges of the mantle make the shell already de- 

 scribed, while the mantle on the posterior part of the 

 body builds the septa during periods of rest. The short 

 funnels of the siphuncle are made by the mantle, while 

 the siphon secretes the porous wall which connects the 

 funnels together. 



In the group of Nautiloids there is no straight old age 

 form, the involute form existing at the present time. 



