75,4 THE POPULAR SCIENCE MONTHLY. 



. The definition of the Cephalopoda, or cuttle-fish class, is largely a 

 matter of commonplace observation. Linnajus, naming them " cepha- 

 lopods," or " head-footed " mollusks, indicated the structural feature 

 which was calculated to appeal most plainly even to non-technical 

 minds. The circlet of arms, feet, or tentacles crowning the head- 

 extremity of a cuttle-fish, thus presents us with a personal character of 

 unmistakable nature. It is necessary, however, to bear in mind that 

 the ordinary and, to a certain extent, natural fashion of representing 

 a cuttle-fish head upward is, in zoological eyes, a complete reversion 

 of its surfaces. To understand clearly why to speak of a cuttle-fish 

 head as its lovjer, and of its tail as its upper extremity, is a correct 

 zoological designation, we must enter upon a comparison of the cuttle- 

 fish body with the forms of its neighbor mollusks. The contemplation 

 of such a familiar being as a snail or whelk introduces us to a charac- 

 teristic example of molluscan form and anatomy. The head of the 

 snail or other gasteropod is clearly enough defined ; and no less plainly 

 discernible is the enlarged and broadened surface on which the animal 

 walks. This surface is known as the " foot." In one shape or another 

 this " foot " is a characteristic possession of the molluscan tribes. In 

 a section of a mussel or cockle, we perceive the " foot " to exist as a 

 muscular mass developed in the middle line of the body below, and 

 variously used in the mussel class as a spinning organ, a leaping-pole, 

 and a boring apparatus. Here we note the natural development of the 

 foot in the middle line of the animal. Let us suppose this foot to be 

 extended downward, and to be broadened so as to form a surface of 

 progression, and we may conceive readily of the modification whereby 

 a simple foot like that of the mussel becomes developed to form the 

 enlarged disk of the gasteropod. In the latter case we observe that 

 the foot occupies the floor of the body ; the bulk of the body, and the 

 head in particular, being borne above. 



Cuttle-fish development can be shown to run, so far, in parallel 

 lines to those of the personal evolution of mussel and snail. But di- 

 vergent paths soon appear in cuttle-fish development ; and these varia- 

 tions, while they indicate an ancient departure from the ordinary 

 molluscan type, likewise give to the subjects of our present study their 

 most characteristic features. When a mussel or snail is watched in its 

 earlier stages of development, the embryo is seen, sooner or later, to 

 produce an . appendage highly characteristic of molluscan young at 

 large, and named the velum. By aid of this ciliated fold such an or- 

 ganism as a young cockle, for instance, swims freely through its na- 

 tive waters. This velum undergoes varied changes and alterations in 

 the after-stages of molluscan development ; but, when cuttle-fish de- 

 velopment is studied in its fullest details, no velum is found among 

 the possessions of the larval body. Such an omission has naturally 

 been made the subject of remark by naturalists. Some authorities 

 Grenacher, for instance have insisted upon the recognition of the 



