I. ORGANISATION OF THE PTEROPODA. 
I propose to study the Thecosomata and the Gymnosomata separately, and as I have 
enumerated the differential characters of these two divisions in the Systematic Report on 
Gymnosomata , 1 there is no need to repeat them here. The ph}dogenetic origin of the 
two groups will be examined in the sequel. 
I. THECOSOMATA. 
We shall now proceed to examine in succession each of the genera recognised in the 
Systematic Report on the Thecosomata . 2 
Family I. LiMACfniDA 
The knowledge of the organisation of the Limacinidas is of the utmost consequence 
in the morphological study of the Pteropoda, because the true phylogenetic relations 
of the typical Thecosomata (Cavoliniidae) are entirely concealed by an adventitious 
modification upon which we shall enlarge hereafter. This masks their true relationship, 
and if the Limacinidae did not exist at the present time it would probably be impossible 
to explain this modification, and consequently very difficult to establish the real affinities 
of the Thecosomata. 
The only information we possess regarding the organisation of the Limacinidae is to 
be found in the memoir of van Beneclen on Limacina arctica ( — lielicina ), 3 and in the 
rather brief anatomical study of the “ genus ” Spirialis, published by Souleyet in the 
Voyage of the “ Bonite .” 4 These two works, however, in addition to being brief, include 
numerous gaps and several inaccuracies, which defects I shall endeavour to remedy. 
Limacina lielicina ( Limacina arctica), the species studied by van Beneden, is of large 
1 Zool. Chall. Exp., part lviii. pp. 4-6. 2 Zool. Chall. Exp., partlxv. 
3 Mdm. Acad. Sci. Bruxelles, t. xiv. 4 Voyage de la Bonite, Zoologie, t. ii. pp. 208-215. 
