METAZOA MOLLUSCA. 261 



part of the dorsal side of the body, but as it develops 

 this shell sac becomes aborted and neither the young 

 (No. 637) nor the adult Octopus (No. 638) have a 

 vestige of a shell. The animal is built upon the same 

 plan of structure as the squid. The model shows the 

 natural position of the Octopus with the mouth downward 

 and long arms extending out on all sides, provided with 

 the sucking discs that make this animal a formidable 

 one. Above is the great rounded head with its prom- 

 inent eyes. 



The alcoholic specimen (No. 639) shows the large open 

 hyponome, the arms surrounding the mouth, and the 

 round plump body. The membrane that connects the 

 base of the long tapering arms and the suckers are well 

 seen in No. 640. The hyponome in this specimen is 

 small. 



In Eledone aldrovandi Delle Chiaje (No. 641), there is 

 but one row of suckers on the arms. Loligopsis verani 

 Per. (No. 642), has a more slender body and two very 

 long delicate arms. 



Argonauta argo Linn., or the Paper Nautilus (No. 643, 

 eggs, 2 females, i male) is related to the Octopus, the 

 structure of the two animals being similar. Here we 

 have no shell comparable with that of other Cephalopods. 

 The male (No. 643, alcoholic specimen ; No. 644, model) 

 is without any protective covering, but the female (No. 

 643, alcoholic specimen ; No. 645, model) has an egg 

 case (No. 646) of exquisite beauty. This case is in the 

 form of a spiral shell, and is developed late in a post- 

 embryonic stage. It is composed of three layers, one 

 made by the edge of the mantle, another by the whole 

 mantle, and the outer layer by the two large arms (see 

 No. 645). The last named layer does not occur in the 

 shells of other Cephalopods. 



The male is very much smaller than the female. One 

 of the arms becomes modified and enclosed in a sac, as 

 seen in No. 644. Later this sac splits and the arm is 



