CEPHALOPOD ADAPTATIONS 



95 



scaphites (fig. 10), the family Lytoceratidae, and 

 numerous others, show a similar elongation of the 

 body chamber. 



Without attempting a detailed explana- 

 tion, it may be noted that in either 

 swimming or passively floating forms 

 there seems to be a direct correlation be- 

 tween the weight of the body and the 

 buoyancy of the empty chambers of the 

 shell, calculated to maintain the animal 

 in a normally functional position accord- 

 ing to its habits. This may seem like 

 reasoning in a circle, but I shall try to 

 show in a few selected cases that it is not. 



Plate z, figure z shows a hypothetical 

 stage in which the shell had become 

 partially elongated and contracted, the 

 visceral cone was correspondingly nar- 

 rowed, and a few partial septa, lined 

 internally with homy endocones, had 

 developed, much as in the family Endo- 

 ceratidae of the Ordovician and Silurian 

 periods, though not in such an advanced 

 manner as in that family. The shell had 

 not yet developed enough air chambers to 

 be appreciably buoyant, and the foot had 

 shortened progressively, although still 

 possessing a partially functional crawling 

 surface; its anterior portion had com- 

 menced to encircle the mouth, and to 

 assume tactile and prehensile functions. 



Plate z, figure 3 shows a second, more 

 advanced hypothetical stage, with a still 

 narrower and more elongated shell and 

 visceral cone, and more numerous septa. 

 The foot is almost entirely transformed 

 into tentacular segments, which com- 

 pletely encircle the mouth; its hinder 

 portion has the two reduced lateral halves 

 appressed, much as in the taxodont genus 

 Nucula, and this part becomes the incipient 

 swimming siphon or hyponomic funnel. 



In Plate z, figure 4 we pass from the 

 hypothetical to the actually observed 

 Orthoceras form with which it is custom- 

 ary to start cephalopod phylogeny. The 



most obvious feature, giving its name to 

 this type, is that the shell is an orthocone, 

 or straight cone. However, a more 

 searching study of the orthocones that are 

 so abundant in the older Paleozoic rocks 

 shows the greatest amount of variety in 

 the details of structure, and is clearly 

 indicative of a like variety of habits of 

 life. When we recall that orthocones 

 swarmed in the older Paleozoic seas, and 

 largely filled the r61e of the fishes of later 

 times, we are bound to admit the prob- 

 ability of their having become adapted 

 for every possible environmental niche. 

 Some were sluggish, others active; some 

 were benthonic, crawling on the sea 

 bottom; others nectonic or swimming 

 forms; and still others may have been 

 planktonic, floating on the surface. They 

 ranged in size from that of a lead-pencil to 

 giant forms a dozen feet or more in length. 

 (Certain species of Endoceras are said to 

 have attained a length of 15 feet.) Their 

 siphuncles were tiny to excessively large, 

 and variously modified; their early cham- 

 bers were empty or filled to various 

 degrees with organic deposits; some fash- 

 ioned accessory chambers far forward on 

 top of the adult living chamber, whose 

 buoyancy enabled them to maintain an 

 even keel (See Plate 3). I shall return to 

 some of these modifications of the ortho- 

 cones and their probable interpretation 

 after following the general course of 

 evolution of the whole group on through 

 to the attainment of the enrolled shells 

 so typical of the late Paleozoic and suc- 

 ceeding Mesozoic era. 



PROGRESSIVE COILING OF THE SHELL 



If one single feature may be said to 

 characterize the phylogeny of the shelled 

 cephalopods as a whole, it is that of pro- 

 gressive coiling. The older naturalists 

 found a supposed reason for this in the 

 phrase "natural selection favored the 



