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



PTEROPODA. 



31 



These two species are Lima- 

 cina helicoides and Clio falcata, 

 the soft parts of which have not 

 previously been investigated. My 

 reasons for considering these two 

 species as archaic representatives 

 of their respective groups were 

 founded first upon the position 

 of their pallial cavities, and later 

 upon the structure or position of 

 nearly every organ system. 



The existence of such archaic 

 species must, of course, influence 

 our view of the relationship within 

 the thecosomatous pteropods, and 

 the whole question as to the posi- 

 tion of the organs in the Lima- 

 cinidae and Cavoliniidae must 

 therefore be subjected to revision. 



Pallial cavity. According to 

 Pelseneer (1906) the right-sided 

 position of the pallial cavity in 

 Peraclis is, like the ctenidium, to 

 be considered a direct inheritance 



from the Bulloidea, the supposed ancestors of the theco- 

 somatous pteropods among the Tectibranchiates. 



My own results with regard to the genus Peraclis 

 have on all essential points supported those of Pelseneer, 

 and I feel fully justified in considering this genus as the 

 most archaic group among the Thecosomata. The right- 

 sided position of the pallial cavity in Limacina helicoides 

 and Clio falcata thus needs no further explanation, being 

 simply an expression of their relationship to Peraclis. 



A comparison between the two species, however, 

 proves a divergence in the development of the pallial 

 cavity, which extends from the right side dorsally in 

 Limacina helicoides and ventrally in Clio falcata. A 

 further divergence in the same directions would result in 

 the directly opposite, dorsal and ventral, position of the 

 pallial cavity in the typical representatives of the genus 

 Limacina and in the family Cavoliniidae. 



Boas (1886) discussed the question as to whether the 

 change of position of the mantle and pallial cavity was 

 perhaps due to a displacement of this part of the body 

 alone, independent of others organs, or to a rotation of 

 the body as a whole, and he came to the conclusion that 

 a complete rotation of the body (relative to the head) 

 had taken place without any essential change in the 

 mutual relations of different organ-systems. 



With regard to the pallial cavity this view was based 

 partly on the existence of a "balancer" protruding from 



Fig. 26. A: one of the Limacinidae. dorsal view. 

 C: Cavoliniidae, ventral view. 



B: Limacinidae. ventral view. 

 (After Boas). 



the mantle-margin on the right side in Limacina but on 

 the left side in certain Cavoliniidae (Creseis), showing 

 that the change of position of the pallial cavity was brought 

 about through an actual change of the right and left sides 

 of the body, and not through a gradual closing of the 

 cavity dorsally and a corresponding increase ventrally. 



I must confess that I have not been able to discover 

 any "balancer", like those of Peraclis or Limacina heli- 

 coides, in any of the Cavoliniidae 1 ) which 1 have had an 

 opportunity of investigating. But under the mantle-margin 

 I found another organ, the lobe-like gill, homologous in 

 Limacina helicoides and all the Cavoliniidae examined. 



This organ does not, however, partake of any rota- 

 tion, it is found in Limacina helicoides in the same place 

 as the ctenidium of Peraclis, i. e. on the right side 

 of the body, and it occupies the same position in the 

 Cavoliniidae. If, therefore, the whole body had rotated 

 with the pallied cavity, then the gill must have rotated 

 independently in the opposite direction in order to main- 

 tain its original position relative to the head. 



An investigation of other organ-systems will, however, 

 give further proofs of an independence between them 

 considerably greater than that supposed in the original 

 rotation-hypothesis. 



Digestive tract. As shown by earlier investigators, 

 the digestive tract in the typical Cavoliniidae is quite 

 opposed to that in a typical Limacina (see textfig. 26 and 



') The specimens of Hyalocylix, Slyliola and Creseis at my disposal did not allow of a thorough investigation. 



