20 THE VOYAGE OF H.M.S. CHALLENGER. 



20. Verticordia tornata (Jeffreys). Station 106 ; 1850 fathoms ; a single specimen. 



The mantle is almost quite open ventrally ; only a short pallial commissure 

 separates the branchial from the jaedal aperture (from q to j, PI. III. fig. 3). The tw(j 

 posterior orifices are surrounded by a common crown of tentacles arranged in a single 

 row (r) and of an odd number ; one of them being alone on the dorsal side, and seven 

 others, of different sizes, on each side. 



The anal aperture has a short siphon (PI. III. fig. 3, ^j). The branchial aperture 

 is sessile. But it possesses, internally, a large tubular valve, incomjoletely closed (cut 

 open on the ventral side) (PL III. fig. 3, k), which certainly cannot be externally 

 evaginated. 



The foot ((/), retracted in the specimen, must be very long in its extended state ; it 

 has a byssal groove on its posterior surface (PI. III. fig. 4, cV). 



The buccal aperture (a) has two pairs of labial palps, the anterior [h) greatly 

 developed, the posterior (c) somewhat reduced. 



There is no gill, or, at least, no structure like the respiratory organs of other known 

 Pelecypoda. We have seen in Lyonsiella (PI. II. fig. 10, c) that the gills form a par- 

 tition separating the pallial cavity into two great chambers, and traversed by the foot. 



Here a similar partition exists [e). But it is muscular throughout its entire 

 length, and is entirely different in structure and aspect from the branchial partition of 

 Li/onsiella. It only exhibits on each side two groups of transverse lamellae, very 

 slightly projecting {g and g'), between which narrow slits establish communica- 

 tion between one chamber and the other. These groups of lamellae exhibit three 

 longitudinal stays on the inner surface of the partition (see PI. III. fig. 5, c). 



The muscular partition extends from the anterior adductor to behind the foot, which 

 traverses it without uniting with it. Behind the foot it is continued, without interrup- 

 tion, by a thinner portion [h), which forms the division between the two siphons. 



The relation and position of this partition place it beyond doubt that it is 

 homologous with the branchial partition in Lyonsiella (see PI. II. fig. 10, e). We have 

 thus here to do with a very remarkable rudimentary respiratory apparatus. The two 

 groups of transverse lamellae on each side (g and g') seem to me to represent a portion 

 of the branchial lamellae which has been preserved. 



The muscular partition is united to the mantle over all its circumference ; at 

 several points on each side there are muscular bundles, which attach it to the shell : an 

 anterior, beside the retractor muscle of the foot ; a second, dorsal and rather large ; 

 and a third, posterior, long and delicate (PI. III. fig. 4, h), which is inserted near the 

 posterior retractors of the foot. These last (n) are remarkable because they are onlj^ 

 distinct at their insertion, and arise from a common trunk. The muscles of the 

 partition probably serve to make it contract in order to expel, through the slits between 

 the plates (g, g'), some of the water contained in the anal or dorsal chamber. 



