36 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



Fig. 30. Head and wings of Limacinidae and Cavoliniidae (somewhat diagrammatic):— 

 A. Peraclis diversa; p. proboscis, op. operculum; B. Limacina helicoides; 1. your"*, 2. full- 

 grown (right half); C. Limacina balea ; B' Clio falcata; C Clio cuspiJata, D' Clio pyra- 

 midata; E' Hyalocylix striata (from Boas); C", 1 — 2, Diacria trispinosa, D" Cavolinia 

 tridentata (from Boas). Figs. B' end C", 1 show the dorsal view, all the others the ventral view. 



This swimming-plate is further developed in the family 

 Cymbuliidae, while it undergoes a retrograde development 

 and a special differentiation in the genus Limacina and 

 the family Cavoliniidae (see textfig. 30). 



The swimming-plate of Peraclis (A) is kept tightly 

 folded during rest, and covered by an operculum carried 

 by the median (ventral) part of the plate. A similar 

 folding of the dorsal (lateral) parts of the swimming-plate 

 is found also in Limacina helicoides (B) and Clio falcata 

 (B'j, but in each of the two groups of which these species 

 are the representatives, the soft and folded dorsal part of 

 the wings is gradually replaced by more rigid wing-like lobes, 

 which during rest cover each other without being folded. 



At the same time the ventral (median) lobe of the 

 plate, which (with or without operculum) originally served 



as an organ of protection for the head 

 and the dorsal part of the foot (see 

 textfig. 30, B'), is in the Cavoliniidae 

 gradually reduced to a small rounded 

 lobe. Initial stages of this reduction 

 are found in Diacria trispinosa and 

 Clio cuspidata (C" and C), which form 

 the bases of two parallel series of 

 development. 



3) The structure of the pallial gland 

 is different in the three archaic forms 

 (Peraclis, Limacina helicoides and Clio 

 falcata), and therefore allows of no 

 general conclusions with regard to its 

 original character. But in the Cavolinii- 

 dae the transverse striation of the gland, 

 so conspicuous not only in Clio falcata 

 but also in Diacria trispinosa and Clio 

 cuspidata, must be considered older 

 than the uniform condition found in 

 Creseis and other forms. 



4) The position of the heart and 

 kidney has played a great part in theo- 

 ries about the relationship between 

 different groups of Cavoliniidae. Ac- 

 cording to Boas these organs, which 

 in the Limacinidae are placed beside 

 the left border of the dorsal pallial 

 gland, should during the rotation of 

 the body have kept their original posi- 

 tion relative to this gland, and therefore 

 in the most primitive Cavoliniidae the 

 heart and kidney should be found near 

 the right side of the body beside the 

 (now ventral) pallial gland. Such condi- 

 tions are found in Creseis, which also 

 for this reason has been considered the 

 most primitive group of the Cavolinii- 

 dae, while the different relations of the same organs in 

 other forms have been considered the results of a second- 

 ary development. 



In Clio falcata, however, we find relations which do 

 not support this view, showing that the original position 

 of the heart and kidney at the left side of the (dorsal) 

 pallial gland may be changed before the gland has full- 

 filled its displacement to the ventral side. In this species 

 the heart and kidney form a transverse girdle over the 

 ventrale side posterior to the pallial gland (pi. II, fig. 13), 

 while the gland still covers the right side of the body. 

 This one fact is sufficient to deprive the phylogenetic 

 considerations based upon the position of the heart and 

 kidney of their value. The reason for the varying posi- 

 tion of these organs should probably be sought not so 



