98 M. Ussow’s Zoologico-Kmbryological Investigations. 
THE CEPHALOPODA. 
No group of Invertebrate animals possesses so igh an 
interest as the Cephalopoda with regard to the complication of 
their bodily structure. And, in fact, since the time of Cuvier*, 
who, taking the exact data of comparative anatomy into con- 
sideration, first sharply defined them and separated them from 
the other classes of Mollusca, they have been placed by most 
zoologists} at the head of all Invertebrata. Some natural- 
istst who wished to see zoological classification founded upon 
embryological facts (at that time still little known and often 
misunderstood) thought that it might be possible to separate 
the Cephalopoda altogether from the Molluscan type, and to 
form a special type of them. Even before this peculiar 
opinion was expressed, a special kind (evolutio radiata §) of 
the so-called unilateral development was established for the 
Cephalopoda and some other Mollusca. Without denying 
the merit of these conceptions as to the systematic position of 
the Cephalopoda in the animal kingdom, which were valu- 
able in their time, we may be allowed to put the question 
whether we are sufliciently acquainted with the most important 
modes of development of the organism of the Cephalopoda, 
and whether we are in a position, resting upon embryological 
facts, to state accurately the most sharply marked traits of 
their phylogenetic connexion, not with all the other types of 
the animal kingdom, but merely with the other classes of 
Mollusea, as with the Gasteropoda, and especially with the 
Pteropoda||. If we look closely into this last highly important 
scientific question, however, it appears that the positive facts 
now known to us regarding the developmental history of the 
Cephalopoda are far from sufficient, even approximately, to 
elucidate their genealogical relations. Notwithstanding the 
interesting results which were to be expected from the’ inves- 
tigation of the developmental history of as many species of 
Cephalopoda as possible, we at present possess only three more 
* Mém. pour servir a hist. de Anat. des Mollusques, 1817, Mém. i. 
+ Lamarck, Hist. Nat. des Anim. sans Vert. 2™° édit. xi. p. 165; R. 
Leuckart, Ueber die Morphol. und die Verwandtschaftsverhiltn. der wir- 
bellosen Thiere, 1848; Huxley, Lectures on the Elem. of Comp. Anat. 
1864, p. 85; Gegenbaur, Verg]. Anat. 2te Aufl. 1870, p.78; Hiickel, Gen. 
Morphol. Bd. ii. pp. exv, 408 et seg. ; Claus, Grundztige der Zool. 2te Aufl. 
1873, pp. 43, 44, 766 et seg. 
t Vogt, Zool. Briefe, 1851, Bd. i. p. 298. 
§ Von Baer, “ Beitr. zur Kenntn. der niederen Thiere,” Nova Acta &e. 
xiii. pl. ii. 1827; Kolliker, Entwicklungsgesch. der Cephalopoden, 1844, 
7D. 
3 || See Leuckart, 7. c. p. 154; Gegenhaur, /. c. p.475; Hackel, Zc. pp. civ, 
exv; Keferstein, Klassen und Ordn. der Weichthiere, p. 1472. 
