CEPHALOPODA. 383 



bodily into the mantle cavity of the female. There its discoverers 

 described it as a parasitic worm " Hectocotylus" The lost arm is after- 

 wards regenerated. In Sepia, however, the modified arm is not dis- 

 charged, but is simply thrust into the mantle cavity of the female. The 

 spermatophores probably enter the oviduct and burst there. 



The eggs when laid are enclosed within separate black capsules con- 

 taining gelatinous stuff, but the stalks of the capsule are united so that a 

 bunch of " sea grapes " results. 



Second Type of CEPHALOPODA. The Pearly Nautilus 

 (Nautilus pompilius). 



The shells of the pearly nautilus are common on the 

 shores of warm seas, but the animals are very rare. Natu- 

 ralists do not seem to know how to get them, though the 

 natives of Fiji and New Hebrides, who appreciate their 

 flesh, trap them successfully in lobster pots baited with 

 crustacean or sea urchin. The animal creeps or swims 

 gently along the bottom at no great depth, and its appear- 

 ance on the surface, "floating like a tortoiseshell cat," is 

 probably the result of storms. It is called " pearly " on 

 account of the appearance of the innermost layer of the 

 shell. This is exposed after the soft organic stratum and 

 the median layer which bears bands of colour have been 

 worn away, or dissolved in a dolphin's stomach, or artificially 

 treated with acid. 



The beautiful shell is a spiral in one plane, divided into a 

 set of chambers, in the last of which the animal lives, while 

 the others contain gas. The young creature inhabits a tiny 

 shell curved like a horn ; it grows too big for this, and 

 proceeds to enlarge its dwelling, meanwhile drawing itself 

 forward in the older part, and forming a door of lime behind 

 it. This process is repeated again and again ; as an addition 

 is made in front, the animal draws itself forward a little, and 

 shuts off a part of the chamber in which it has been living. 

 The compartments seen on a divided shell are not exactly 

 successive chambers, they are fractions of successive cham- 

 bers abandoned and partitioned off as more space was gained 

 in front. Moreover, all the compartments are in communi- 

 cation by a median tube of skin the siphuncle which is 

 in part calcareous. 



It has been suggested, that " each septum shutting off an 

 air-containing chamber is formed during a period of quies- 



