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



PTEROPODA. 



11 



with a sharp and curved ridge, which corresponds with 

 a keel on the outside: umbilicus none. (See texfig. 6). 



With regard to the animal the following points 

 are of importance: Head indistinct, consisting of a 

 sligthtly protruding horseshoe-shaped lip on the dorsal 

 side of the mouth. Tentacles asymmetrically developed, 

 the right one considerably larger than the left and sur- 

 rounded by a sheath (textfig. 7). Both tentacles have the 

 dark colour of the wings but with ivory-coloured free ends. 

 With regard to the nature of these convex ivory-coloured 

 end-plates of the tentacles (see also pi. I, fig. 4 t) I can 

 as yet give no definite information. At the first glance 

 they look like eyes, but it would perhaps be just as 

 appropriate to consider them light-organs. They con- 

 sist of big clear cells with large nuclei, and seem abso- 

 lutely destitute of pigment. I hope in a later paper to 

 deal with the finer structure of this organ. 



The r a d u 1 a (textfig. 8) is very small in proportion 

 to the large size of the animal (comp. textfig. 5 and 8 B, 

 drawn on the same scale); all teeth are, with the exception 

 of their clear colourless points, dark brown like the 

 pharynx itself. Median and lateral teeth are very much 

 alike as to size and shape. A very faint rudiment of the 

 second lateral tooth found in Peraclls is present also in 

 this species. 



The foot is developed into two large wings (pi. I, 

 fig. 4, 6), forming together a swimming-plate without any 

 distinction between lateral and posterior lobes (fig. 5). 

 When at rest the wings scarcely or never cover each 

 other, but are regularly and symmetrically folded (fig. 7, 9) 

 like those of Peraclls (fig. 2), so as to fill the whole 

 dorsal part of the shell-mouth. Ventrally this mouth is 

 occupied by a folded, lobe-like projection from the under- 

 side of the body (fig. 5, v. b). 



Operculum none. 



The pallial cavity is placed on the right side 

 (pi. I, fig. 6), following this side from the median dorsal 

 line of the body until not far from the same line on the 

 ventral side. The pallial gland (fig. 6 — 7 p. g) is 

 uniformly developed, without the transverse folding of 

 Peraclls (fig. 2, p. g). 



The mantle-margin (fig. 4, 6 m. m) is bordered 

 by a very thin membrane, under which a pointed thread- 

 like balancer protrudes on the right side of the body 

 (pi. I, fig. 5, pi. Ill, fig. 24, bal.). At the side of this 

 balancer we see a semicircular membranous lobe 

 (fig. 5, 6, 24, g) which probably ought to be considered 

 a respiratory organ. The position of this lobe, at 

 a place corresponding to that of the ctenidium in 

 Peraclls, and just fitting into the siphon-like curve of 

 the shell (fig. 5), makes this in fact so probable, that I 

 feel justified in regarding this lobe, which is found also 



in other species, as a gill, analogous if not homologous 

 to the ctenidium of Peraclls. 



Heart and kidney are found posteriorly at the 

 left side of the pallial gland (fig. 7, h). 



The course of the intestine may be followed in 

 fig. 7 and 9, and will be found diagrammatically demon- 

 strated in textfig. 27. From the stomach (st. fig. 8, 9), 

 in which the unpaired tooth has a dorsal position, the 

 intestine (fig. 9 i 1 ) runs obliquely towards the right 

 side along the surface of the liver (/.), passing below the 

 rectum (/-.), making a dorsal loop towards the left side 

 of the animal (fig. 7 i 2 ) and back again (fig. 9 i 3 ), — the 

 rectum finally runs forwards along the right side of the 

 body where the anus (fig. 9, a) is situated at the side 

 of the stomach. 



The visceral and abdominal ganglia are 

 united to one symmetrically developed mass, 

 from which the abdominal nerve runs towards the right 

 (see textfig. 28). 



The genital duct (fig. 9 g. d.) runs forwards from 

 the genital gland at the posterior end of the body parallel 

 to the rectum and opens on the right side of the neck. 

 The male and female organs of one individual become 

 developed at different times, and the difference between 

 "male" and "female" individuals is very obviously demon- 

 strated in figs. 9 and 10. In fig. 8 and 9 the male organs 

 are fully developed; the penis is evaginated and the 

 groove, which leads from the genital opening {g. o.) may 

 be followed all along the border of the penis to the top 

 of the finger-like protrusion at its distal end. Fig. 10 on 

 the other hand shows a "female" in which the uterus 

 (u) is filled with a series of large spheres, each containing 

 a fullgrown larva (fig. 11). As will be seen from the above, 

 L. helicoldes is viviparous. 



Colour: Wings, head, tentacles and mantle-margin 

 are more or less dark, velvet-like, brown, this dark colour 

 being interrupted only by the ivory-coloured ends of the 

 tentacles (fig. 4, 6). 



The shell is of a shiny bronze colour. 



The dimensions of this species far exceed those 

 of all other known Limacinidae, the shell reaching a 

 breadth of about 10 mm., while the largest individuals 

 measure about 15 mm. across the wings. 



As will be seen from the above description, L. heli- 

 coldes, which on account of its asymmetrically developed 

 tentacles and the whole arrangement of its internal 

 organs must be considered a true Limaclna, differs at 

 the same time from the typical species of this genus in 

 certain interesting points. 



The development of the foot into a broad undivided 

 swimming-plate, as well as its folded position during rest, 



