36 MR. E. RAY LANKESTER ON THE 
have the necessary conditions for the growth of such a “pen” as that of the Decapo- 
dous ** Cephalopods.” 
At present the details of the development of the “ pen” in the Cephalopoda are not 
fully known *. I have evidence that it is formed in an enclosed sac-like diverticulum 
of the epidermis, but have not yet ascertained the earliest condition of this sac. ‘The 
history of its development becomes surrounded with additional interest in relation to 
the shell-gland of the other Mollusca. 
The position of the groove of the shell-gland in Pisidiwm suggests a possible con- 
nexion of its chitinous plug with the ligament, which it will be worth inquiring into 
in other developmental histories of Lamellibranchs. 
In Dentalium and Fissurella it appears to be exactly that region of the shell which 
would correspond with the first-produced chitinous shell-disk and its plug, which is 
altogether absent, leaving an open hole. 
_ The internal shells of other Mollusca besides the cuttlefish are certainly not in 
some cases (e. g. Aplysia) primitively internal, but become enclosed by overspreading 
folds of the mantle. But in the case of imax and its allies, it is possible (though 
requiring renewed investigation) that the shell is a primitively internal one, representing 
the shell-plug. 
There is yet one more possible connexion of this shell-gland and plug: this is the 
chitinous secretion by which Terebratula and its allies fix themselves to rocks &c. The 
position of the peduncle exactly corresponds to that of the shell-gland; and an exami- 
nation of Professor Morsr’s recently published account of the development of Terebra- 
tulina, leaves little doubt that at the pole of attachment, which very early develops its 
function and fixes the embryo, an in-pushing occurs, and a kind of shallow gland is 
formed, which gives rise to the horny cement. My own observations on the develop- 
ment of Terebratula vitrea do not extend to so early a period as this. 
It is perhaps scarcely necessary, in conclusion, to point out the close resemblance of 
shell-gland and plug to the byssal gland and its secretion. They are closely similar 
structures; but there does not appear to be any reason for regarding them as “serial 
homologues,” or as more closely related than are, say, the hairs on the head of a man 
with the hairs on his chest. 
Explanation of the lettering of Plates 5, 6, 7,8, 9,10. (For explanation of the lettering 
of Plates 1, 2, 3, 4 see page 12.) 
al. Alimentary canal. 
C. primitive gastric cavity. 
c. Body-cayity or ccelon. 
ci. Cilia. 
* March 7th, 1875.—I may be permitted to refer to two papers published since the above was written, 
in which my subsequent observations are related establishing the mode of origin of the Cephalopod’s pen-sac as 
an open pit. They are contained in the Quart. Journal of Microsc. Science, October 1874 and January 1875. 
