REPORT ON THE PTEROPODA. 
89 
which has already undergone numerous modifications, and has become widely separated 
from the primitive symmetrical Archimollusc. 
B. The concentration of the nervous centres indicates a very specialised and highly 
differentiated group. 
2. In the course of their development the Pteropods pass through a stage even more 
asymmetrical than the adult. This fact indicates clearly that they arise from ancestors 
more asymmetrical than themselves, and that their apparent symmetry has been 
acquired in the course of time by adaptation to their natatory habits. 
Comparative anatomy and embryology indicate then that the Pteropoda are not 
primitive Mollusca, and furthermore, that they are derived from ancestors which them- 
selves are not primitive, but on the contrary already specialised. Some naturalists 
entirely misunderstand the degree of specialisation of the Gastropoda ; thus Boutan 1 
regards as the most primitive those Gastropods which he calls typical, that is to say those 
in which the asymmetry is carried to the highest pitch, and he criticises the opinion of 
Spengel, who regards the Fissurellidse as primitive Gastropoda. Among the asymmetrical 
Gastropoda, Fissurella and its allies are in fact the most primitive, as is shown by the 
conformation of some of their organs (e.g., those of circulation and excretion) ; such is 
the inaccurate point of view which Boutan has adopted and from which he has been led 
to confound the judicious conclusions of Spengel with the rash generalisations of von 
Jhering. 
3. The organisms from the Primary formations, which are usually referred to the 
Pteropoda, have no affinities with these latter, as I shall show further on. In the 
Secondary rocks there are no traces of Pteropods, the first undoubted remains of this 
group being found in the lower Tertiaries. They are then of recent origin. 
We are consequently justified in saying: — The Pteropoda do not form a primitive 
group, but on the contrary a recent and specialised one — a terminal group. The greater 
part of the characters of terminal groups, as formulated by my esteemed teacher, 
Professor Giarcl, 2 are entirely applicable to the Pteropoda : — 
(1) They are profoundly modified in adaptation to a special mode of existence. 
(2) They exhibit very slight variability. 
(3) They include only a small number of species. 
VI. POLYPHYLETIC ORIGIN OF THE PTEROPODA. 
We regard it then as proved that the Pteropoda (both Thecosomata and Gymno- 
somata) are derived animals and of recent origin, and by no means primitive 
Mollusca. 
1 Recberches sur l’anatomie et le ddveloppement de Fissurella, Archives cle Zaol. Expdr., sdr. 2, t. iii. bis, pp. 150, 15] . 
2 Observations . . . (sur les mammiftres ovipares), Bull. Scient. Depart. Nord, 1886, p. 416. 
(ZOOL. CHALL. EXP. — PAKT LXVI. 1888.) 
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