PHYLUM MOLLUSC A 199 



the sensory cells. In Nautilus, much simpler eyes are found, 

 for lens, vitreous body, cornea, and iris are wanting. 



Two kinds of hearts are present. The systemic heart is of 

 typical molluscan type with its single ventricle and two auricles 

 corresponding to the number of gills and receiving blood from 

 them, but in addition to this heart there is a branchial heart at the 

 base of each ctenidium which forces the blood through the gills. 



Cephalopods are of importance especially because of the part 

 which they play in food chains. Squids and devilfish feed upon 

 fishes and in turn serve as food for still larger fishes and for the 

 sperm whale. They are used extensively for bait and in many 

 regions as food for man. 



The sexes are separate. The spermatophores of the male are 

 frequently stored in arms which become more or less modified. 

 In most instances, this modification involves only sufficient 

 change to adapt the arms as accessory copulatory organs but in a 

 few genera the entire arm bearing the spermatophores becomes 

 severed from the body of the male and acquires independent 

 powers of locomotion. When first observed, these wormlike 

 castaway arms were thought to be entire organisms and were 

 described under the name Hectocotylus before their relationship 

 to the cephalopod body was understood. The term hectocotyli- 

 zation is used to indicate this type of spermatophore transfer 

 through the agency of dissevered tentacles. The heavily yolk- 

 laden telolecithal eggs undergo partial discoidal cleavage. The 

 organs of the young cephalopod are formed from the blastoderm; 

 first as flattened projections, but as these grow the young animal 

 becomes recognizable, at first appended to the bulky yolk sac by 

 the head end. 



Largely on the basis of the number of gills, two subclasses of 

 cephalopods are recognized, the Tetrabranchia with four gills and 

 the Dibranchia with two. 



Subclass Tetrabranchia 



The genus Nautilus contains the only living representatives of 

 the Tetrabranchia. A well-developed, chambered shell within 

 which the animal lives, the presence of four gills, four auricles, 

 and four nephridia, a divided siphon, and many tentacles without 

 hooks or suckers characterize the living examples of this subclass. 

 In past geological ages, numerous tetrabranchs flourished, the 

 fossil shells of which show many interesting evolutionary ten- 



