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



PTEROPODA. 



21 



Very few specimens of this species were previously 

 available; the "Michael Sars" expedition brought home 

 eight complete individuals. The soft parts of this interesting 

 form are very little known, and I shall therefore point 

 out the most characteristic and systematically interesting 

 traits of its anatomy. 



The shell (textfig. 20) well known from the description 

 of Pelseneer (1888), is distinguished from that of nearly 

 related species (for example C. Andreae, Boas) by its want 

 of longitudinal or transverse striae or any other sculpture, 

 except the two slowly diverging lateral carinae. 



About the soft parts of the animal nothing is known 

 except its colour, which is "dunkel-schwarz-violet" (Pfeffer 

 1880) and the similarity of its "foot" to that of C. Andreae, 

 Boas, which according to Munthe (1887) represents a 

 transition-form between the narrow tongue-like foot of 

 Cleodora (Clio) and the broad foot of Hyalaea (Cavolinia). 

 To this I can add the following details: 

 The head of Clio falcata forms a very distinct 

 triangular lobe pointed at its free end. The mouth is 

 situated below this lobe. 



Tentacles asymmetrically developed, the righttentacle 

 being considerably larger than the left one, covered by 

 a sheath and placed more anteriorly (fig. 12, 18 t). Both 

 tentacles are cylindrical with convex ivory-coloured 

 end -plates, like those of Limacina helicoides. 



Foot forming a large continuous swimming-plate, 

 the dorsal (lateral) part of which is soft and during rest 

 tightly and regularly folded on both sides of the triangular 

 lobe (fig. 12, 18). This soft part of the foot is during 

 rest protected by the more rigid ventral lobe (fig. 13 v.l.), 

 which is like an operculum closing the shell-mouth. So 

 far as I have been able to see the margin of the foot is 

 quite continuous, without any incision or lobe-formation. 

 The mantle and the pallial cavity are as in all 

 the Cavoliniidae found on the ventral side of the body 

 (fig. 13), the beautifully coloured transverse striae of the 

 pallial gland attracting attention, but while the stripes 

 of this gland take their origin along the lateral line of 

 the left side, on the right side they continue on to the 

 back of the animal (fig. 12 p.gl), where the mantle-border 

 forms a very conspicuous bow touching the median line 

 of the body. The pallial cavity therefore in Clio falcata 

 occupies not only the ventral but also the right side 

 of the body. At the same time this cavity is very deep, 

 covering nearly the whole length of the ventral side. 



The mantle- margin is, like that of Limacina 

 helicoides, formed by a thin membrane (fig. 13, 20 pi. 

 II, fig. 25 pi. Ill), inside of which a tongue-like lobe 

 protrudes, homologous to the gill of L. helicoides. This 

 gill is in Clio falcata as in L. helicoides found on the 

 right side of the body. 



No balancer was found in this species. 



Heart and kidney are seen to form a transverse 

 row across the ventral side just behind the pallial gland 

 (fig. 13 h.k.). 



After removal of the mantle the position of the 

 viscera is seen as shown in pi. II, fig. 16 — 17, and after 

 a further dissection as in fig. 19. The relations of the 

 digestive tract may be traced in the same figures. 



Fig. 20. Shell of Clio (Euclio) falcata (from Pelseneer 1 



The buccal mass and the passage of the oesophagus 

 through the ring formed by the central nervous system 

 (fig. 19) do not show any peculiarities, but just behind 

 this passage the oesophagus describes half a spiral 

 turn so that at its posterior end the originally ventral 

 side is placed dorsally (fig. 19). 



The stomach of course follows the posterior end 

 of the oesophagus. It occupies the left side of the body 



Fig. 21. Transverse row of the radula in Clio falcata. 



(fig. 16 st.) and its unpaired tooth which (with an 

 untwisted oesophagus) would have been found in the 

 median dorsal line, is placed ventrally (fig. 19 st.t.). 



The intestine leaves the stomach in a right-sided 

 direction, soon however turning to the left (fig. 16), then 

 passing from the dorsal side round the left edge of the 

 liver (/ fig. 16—17) and making a long loop over the 

 ventral and right side of the body (2 and 3 fig. 16—17). 

 After having thus twice crossed the ventral side it once 



