ATLANT. DEEP-SEA EXPED. 1910. VOL. Ill] 



PTEROPODA. 



35 



and the change of position of the body-organs must be 

 looked upon not as a rotation of the body as a whole 

 but as a series of parallel changes going on within each 

 organ-system. 



But although the independence of the organ-systems 

 during this development from Peraclis to the typical 

 Cavoliniidae seems to be much greater than supposed in 

 the original rotation-theory, yet there certainly exist some 

 causal connection between the parallel changes undergone 

 by each of them. 



The primary cause of all these changes going on in 

 the Cavoliniidae may probably be found in the erection 

 of the shell and its influence on the state of equilibrium 

 of the pelagic animals, a result of which is seen in their 

 peculiar mode of swimming with the ventral side upwards. 

 In intimate connection with the change of equilibrium a 

 displacement of the pallial cavity to the ventral side 

 would follow, a free and convenient entrance of the water 

 into this cavity being of vital importance to the animal, 

 while the retractor muscle would withdraw to the oppo- 

 site (dorsal) side. The gill, however, being placed on the 

 right side of the body is equally convenient whether the 

 dorsal or ventral side is turned upwards, and does not 

 change its position during the course of development 

 from Peraclis to the Cavoliniidae. 



As in the case of other molluscs, the position of the 

 pallial cavity is in some way or other decisive also for 

 the position or course of other organ-systems. The 

 stomach which lies between the retractor muscle and the 

 pallial cavity must as a matter of convenience turn its 

 more convex side (carrying the unpaired tooth) towards 

 this cavity; a rotation of the stomach and the digestive 

 tract is therefore a necessary consequence of the displace- 

 ment of the pallial cavity. 



This rotation is (in Clio falcata) brought about 

 through a spiral twisting of the oesophagus, which during 

 its change of course is accompanied by the abdominal 

 nerve, and this probably initiates an asymmetrical develop- 

 ment of the visceral gangliar mass, contrary to that of 

 the genus Limacina. 



The genital gland is in its position so much dependent 

 on the shape of the liver, that a rotation of the whole 

 digestive tract would probably influence the position 

 of this organ also. As mentioned above (p. 22) I have 

 not been able to follow the course of the genital duct 

 in Clio falcata, but the shape and position of the genital 

 gland seem to indicate that the duct takes its origin on the 

 left side of the body, so that the relations characteristic 

 of the Cavoliniidae are here already established. 



\ t)i.acrta. 

 1/ CW*e*in 



CUo 



StyLLoli 



Our view of the relations between the two families 

 Limacinidae and Cavoliniidae, and between the different 

 groups of the Cavoliniidae must be influenced by the 

 knowledge recently acquiered regarding the still living 

 archaic representatives of the groups. 



Starting .from the view that the whole rotation- 

 process was effected in times past, modern investigators 

 (Tesch, Meisenheimer) have followed Boas and Pelseneer 

 in considering as the most primitive Cavolinia. 

 Cavoliniidae a group of species (Cre- 

 seis) in which the position of all the 

 visceral organs is practically opposite 

 to that in the typical species of 

 Limacina. 



"Als Ausgangspunkt stellt sich 

 die Gattung Limacina dar, aus ihr 

 ging unmittelbar unter Streckung des 

 Rumpfes und Drehung desselben um 

 180° die Gattung Creseis hervor." 

 (Meisenheimer 1905, p. 172). 



A further grouping of the genera 

 belonging to the family Cavoliniidae 

 is based upon the existence in each 

 genus of what are considered primitive 

 characters, such as: — 



1) a circular transverse section 

 of the body; 



2) a Limacina-\\kt shape of the 

 wings; 



3) a uniformly developed pallial gland; and 



4) above all a position of the heart and kidney in 

 relation to this gland corresponding to that found in 

 Limacina. 



The most modern treatment of these questions is 

 given by Meisenheimer, who expressed his view of the 

 phylogenetic relations of the genera in the scheme repro- 

 duced in textfig. 29. 



As to what characters are to be regarded as primitive, 

 our recent knowledge of the archaic species may afford 

 some definite information. 



1) A circular transverse section of the body is found 

 neither in Limacina helicoides nor in Clio falcata, and 

 there seems to be no reason for considering it an archaic 

 character. 



2) With regard to the shape of the wings my results 

 are directly opposed to the view maintained by earlier 

 investigators. According to my results the archaic form 

 of the foot of thecosomatous pteropods is not the narrow 

 and well defined wings of a typical Limacina, but the 

 broad continuous swimming-plate of Peraclis, which is 

 present also in Limacina helicoides and Clio falcata, as 

 well as in the genus Procymbulia. 



V 



Cr-eseis 



I 



LiitKLeinct. 



Fig. 29. 

 Phylogenetic relations 

 of the genera of the- 

 cosomatous pteropoda, 

 according to Meisen- 

 heimer. 



