368 KEPOKT OF COMMISSIONER OF FISH AND FISHERIES. [158] 



from the sides of the mantle (v, vc') : one of them (vc'), receiving a 

 branch from the gill, runs from the* anterior part backward; the other 

 (#'), from the middle part forward ; these unite into one trunk before 

 reaching the vencecavce. The posterior vence-cavw (vc") arise mostly 

 from the caudal fins, but receive branches from the posterolateral por- 

 tions of the mantle ; each one receives two large branches, one anterior 

 and the other posterior, just at the point where it leaves the inner sur- 

 face of the mantle. From this point they run forward, parallel with the 

 two posterior arteries, and converge to the region of the heart, where 

 they join the great sacculated venous vessels; along a considerable 

 portion of their course they expand and become large, elongated, fusi- 

 form organs (r"), probably renal in function, but much firmer, more 

 definite in form, and finer in structure than the more anterior renal 

 organs. 



The gills (g) are long, triquetral, acute; in section they are nearly tri- 

 angular (PI. XXXII, fig. 3), with the free ventral sides convex, and the 

 dorsal side flat or concave, except along the middle, where a thin me- 

 dian membrane (d) arises from a central ridge and unites the gill to the 

 inner surface of the mantle. The gills are composed of large numbers 

 of thin, transverse branchial laminae (Fig. 3, a), which extend outward 

 symmetrically, on each side, from the large median blood-vessels (bo r 

 bv), each half of a lamina having a lung ovate or elliptical outline. A 

 somewhat firm central axis or column (c) gives support to the laminae 

 and the large blood-vessels. The gnat afferent vessel (bv) starts from 

 the branchial auricle and runs along the median dorsal side of the gill, 

 on theiuner edge of the axial column (c) ; another parallel venous trunk 

 or sinus (v) is seen near the dorsal edge of the column. Each branchial 

 leaf receives from the afferent vessel (bv) a branch (b) which runs along 

 the dorsal edge, giving off at regular intervals small transverse paral- 

 lel branchlets, which in turn give oft' minute capillary vessels along their 

 sides and fade out near the ventral border of the lamella'. Parallel 

 with these arise small capillary efferent vessels, which join larger trans- 

 verse vessels between and parallel with the afferent ones; these in turn 

 join the larger efferent vessel that runs along tie' ventral edge of the 

 lamina, and these marginal vessels pour their contents into the large 

 vessel (bo) which runs along the middle of the gill on the ventral side 

 and carries the purified blood to the heart. 



The alimentary tract is represented in a nearly dorsal view in Plate 

 XX, fig. 2. In this figure the pharynx is shown in longitudinal section 

 in a side view. The buccal membrane (bin); the pharynx with its horny 

 jaws (sm the superior, and im the inferior mandibles); the odontophore, 

 (od) armed with seven rows of recurved teeth on the radula; and the 

 thin chitinous lining membrane, which bears numerous sharp, scattered, 

 recurved teeth, both on the palate and in the throat, have already been 

 described (pp. 134, 135). The oesophagus (oe) is a long, narrow, but dilat- 

 able tube, having two oblong salivary glands (sg) attached to it just at the 



