34 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



Fig. 28. Nervous system of thecosomatous pteropods seen from the ventral side (somewhat diagrammatic): A. Peraclis 

 (from Tesch 1S04); B. Cymbulia (from Pelseneer 1888); B' Limacina helicoides c. gl. ceretral ganglion; C Limacina 

 helicina (from Tesch); D' Limacina inflata (from Meisenheimer 1905); B" Clio falcata; C" Clio cuspidata; D" Clio 

 pyramidata; E" Clio sulcata (from Meisenheimer); F" Hyalocylix striata (from Meisenheimer); C" Diacria trispinosa 

 (from Meisenheimer); D'" Cavolina tridentata (from Meisenheimer). 



asymmetrically developed ganglia, distinct from, yet in 

 broad connection with, each other. Similar conditions 

 are found in the family Cymbuliidae (B), the only diffe- 

 rence consisting in the deeper constrictions between the 

 three ganglia. 



A development in the opposite direction is found 

 when passing from Peraclis to Limacina helicoides 

 and Clio falcata, for in these two species the three 

 visceral (and abdominal) ganglia coalesce into a single 

 symmetrically developed mass without any constrictions 

 (B', B"). A difference between them is seen, however, 

 in the course of the abdominal nerves, which run obli- 

 quely to the right in Limacina helicoides and to the left 

 in Clio falcata. This difference in the course of the 

 abdominal nerves harmonises with the corresponding 

 difference in the course of the oesophagus in these two 

 species (already referred to), and very probably therefore 

 some causal connection exists between the changes going 

 on within the two organ-systems: the digestive tract and 

 the nervous system. 



Now taking the two species Limacina helicoides and 



Clio falcata as the points of departure we shall find in 

 each group a gradual increase of the asymmetry first 

 shown in the course of the nerves only, the visceral 

 gangliar mass being more developed on the right side 

 in Limacina (C D ) and on the left side in the family 

 Cavoliniidae (C"-F", C'"— D'"). 



The existence at the present day of archaic thecoso- 

 matous pteropods, and the relations of their different 

 organ-systems as demonstrated above, cannot fail to influ- 

 ence our view of the phylogenetic relationship between 

 the more modern representatives of the group. 



The rotation-theory of Boas is supported by the new 

 facts, in so far as a development of the Cavoliniidae from 

 the Limacinidae is shown to have taken place and to 

 have been characterised by a rotation (of 180°) of the 

 main organs of the body relative to those of the head. 



The whole development is, however, shown to have 

 taken its origin not in the modern Limacina-type, but in 

 an archaic type still represented by the genus Peraclis, 



