48 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
best specialized and separated. In the special description of Atlanta names have 
been given to these parts, whose appropriateness is I hope obvious. 
From this highly developed condition of the foot, to its diminished state in Glaucus 
and its total absence in Phyllirrhoe, we have every gradation. The various porUons 
are still to be distinguished in Pteroceras and the Strombidw, but lose their distinct- 
ness for the most part in other Gasteropoda. However, the propodiuin is still 
marked off by a transverse fissure in Oliva and Ancillaria, and attains a great deve- 
lopment in size ; a peculiarity which is still more remarkable in Natica and Sigaretus. 
In both these genera it shows a tendency to invest the head. In the Cephalopoda, 
the anterior arms, which must be considered as the propodium, fairly unite in front 
of the mouth, and it seems very possible that the cephalic hood of Gasteropteron, the 
“ orar tentacles of Aplijsia, the hood of Tethys, the - lips” of some Pteropoda, and 
the hood of Pneumodermon may be the result of a similar change. But all attempts 
to settle these points, save by development, must be more or less hypothetical. 
To this same test of development we must refer everything which claims to be 
called ‘^mantle;' a word which has perhaps been more vaguely and loosely used than 
any other term in zoology. Surely if a term is to have any value in either zoological or 
anatomical nomenclature it must be applied to only a defined thing. The “ mantles 
of a Sepia, a Cleodora, or a Buccinuni may be homologous with one another, but 
they certainly are not so with what is called the “ mantle” in Doris or any other 
Nudibranchf. The simple fact that the cephalic tentacles arise in the midst of the 
is never permanently developed, and the position of the alse would lead to the belief that they correspond to its 
lateral portions’. r i, a u i 
So far as I can judge from the Latin table affixed to his Swedish essay on the development of the Acephala, 
Loven considers the arms of the Cephalopoda, the three pairs of cephalic tentacles of Clio, and the cephalic 
lobes of Tethys, to be homologous with the velum of the Gasteropod embryo, while the funnel of Cephalopoda 
and the alee of Pteropoda are homologous with the foot of Gasteropoda. 
The considerations above cited appear to me to furnish a sufficient refutation of these views, which seem 
to be the offspring of an idea first propounded by their learned author in his “ Contributions to the Embryology 
of the Mollusks” (Oken’s Isis, 1842), that the hood of Tethys and the cephalic processes of Tergipes are modi- 
fications of the cephalic vela of the embryo. This ingenious hypothesis has not however been confirmed by 
observation so far as regards Tethys, and has been directly ^proved with respect to Tergipes (see Schulze, 
Ueber die Entwickelung d. Tergipes lacinulatus, Wiegmann’s Arch. 1849). 
* In the absence of any knowledge of development perhaps the source of the nervous supply is one of the 
best tests of the real homology of a part. Mr. Hancock, in his valuable paper “ Upon the Olfactory Apparatus 
in the Bullidce” (Annals of Nat. Hist. March 1852), has, I observe, applied this test to the cephalic expansion 
of the Bullidee, to the hood of Gasteropteron, &c. ; and since it clearly appears that these parts are supplied by 
nerves from the cephalic ganglia, which never give branches to any portion of the foot, the suggestion in the 
text must be given up. 
t Leuckabt, believing the haimal tegument of the Nudibranchiata to represent a mantle, suggests that there 
’ The researches of Vogt, already referred to, have fully confirmed this conclusion. The embryo Pteropod 
has vela, which eventually disappear, while the epipodia are developed quite distinctly from the upper part of 
the sides of the “ foot.” 
