FESTSKRIFT FÖR LILLJEBORG lo 



the curve of the shell, and if we forcibly curve the posterior end of the body of 

 any other Cephalopod with short fins in a ventral direction we shall get a similar 

 position of the fins. The fins of Spirula are thus, in reality, not inserted differently 

 from those of other Cephalopods and the opinion of Pelseneer (1. c. p. 10) that: 

 "they are thus to the fins of other Dibranchiatcs much as the tails of fishes arc to 

 those of Cetaceans", is of course a mistaken one. The error may possibly be explained 

 by the direction of the fins on the Challenger- and Blake-specimens of Spirula, in 

 which, to judge at least by the figures given, the fins are more or less vertical and 

 extend backwards. But this direction is, I venture to think, only secondary, caused 

 in preserving the flabby fins of the dead animal. In our specimen, which I think 

 displays the normal structure, the fins are curved towards the body, showing an ex- 

 rior convex and interior concave surface (Figs, i, 2, 3). This is the position in 

 which the fins must be held when the animal is swimming backwards, otherwise 

 they would completely hinder progression. The dimensions of the fins are about 

 6 mm. in one direction 3' -1 in the other; the first of these measurements is along the 

 axis parallel to the line of attachment, the second perpendicular to the same. The 

 longest diameter of the fin is thus 28 °/o of the length of the mantle (on the dorsal 

 side) in our specimen of Spirula reticulata, and the fins are thus larger in this species 

 th. in in Spirula pcronii; for Huxley's figure of it gives the corresponding diameter 

 of the fin as 24 7o of the length of the mantle; and in Spirula austräte the fins are 

 much smaller even. The surface of the fins is perfectly smooth, not reticulated. 



The superficial furrow between the true mantle and the "terminal disk" is 

 conspicuous (with a magnifying glass) all round, especially so on the ventral side. 

 The: terminal disk is thick and fleshy and carries in the middle of its distal part the 

 "aboral fossa". This peculiar organ is a round opening which leads to a much larger 

 inner cavity in the centre of which is a white conical papilla. This papilla seems to 

 be smaller in this species than in Spirula pcronii. The disposition of the pigment 

 round this organ has already been mentioned. Fig. 4 gives a picture of the aboral 

 end of our Spirula reticulata with the "terminal disk" and the "aboral fossa". The 

 function of this organ is very questionable, and different authors have expressed dif- 

 ferent opinions about the same. The earliest hypothesis regarded it as an adhesive 

 apparatus, another as a gland (Gray) 1 or mucous pore, while yet another supposed 

 it to be a secondary sexual apparatus which "might serve for the attachment of the 

 cluster of ova after their extrusion" (Owen) ~. These theories have however all been 

 abandoned as it has been proved that it has neither glandular nor acetabular structure, 



1 Ann. Mag. Nat. Hist. 1845, P- 2 59- 

 •' Zool. Voy. ''Samarang - ' 1. c. p. 15. 



