THE SCAPHOPODA 199 



flattened and devoid of palps (Fig. 183, VI). Laterally and pos- 

 teriorly it is provided with two pouches, and quite at its posterior 

 end, on either side of its dorsal surface, are two broad, symmetrical, 

 and flattened tentacular lobes (Fig. 183, IV) which appear to be 

 homologous with the cephalic lobes of Rhipidoglossa (Fig. 130, II). 

 The cephalic filaments or "captacula" (Fig. 181, E) are inserted 

 on the margins of these lobes, and when extended, radiate in all 

 directions from them (Figs. 182, ca; 183, I). 



These captacula are of unequal length, autotomous, and capable 

 of regeneration : they are ciliated, highly contractile, and their 

 extremities are swollen and club-shaped, with a small lateral con- 

 cavity in each. These organs are in the first instance tactile, but 

 also prehensile. The difference in their length is the result of 

 their regeneration after being lost. 



The foot has the form of an elongated cylinder, is very 

 extensible, and when forcibly projected beyond the aperture of the 



FIG. 182. 



Diagram of the organisation of Dentalium, left-side view, o, anus; ca, captacula; c.g, 

 cerebral ganglion ; /, foot ; go, gonad ; in, intestine ; k, left kidney ; la.c, labial commissure ; 

 li, liver ; m, mouth ; o, orifice leading into the perianal sinus ; of, oesophagus ; JM, mantle ; 

 p.g, pedal ganglion, with otocyst ; pl.g, pleural ganglion ; po, posterior orifice of the mantle ; 

 ra, radular sac ; st.g, stomato-gastric ganglion. 



shell, serves as a digging organ. In the Dentaliidae it is pointed 

 in front, but has an oblique wing-shaped fold or pleat on either side 

 of its free extremity : these two folds are contiguous on the ventral 

 but interrupted on the dorsal side. In the Siphonopodidae the foot 

 ends in a retractile disc with papillated margins (Fig. 183, VIII), and 

 in Pulsellum there is a filiform tentacle in the middle of the disc. 

 The powerful retractor muscles of the foot form two symmetrical 

 bundles inserted far back on the dorsal side of the shell (Fig. 181, c) 



II. ANATOMY. 



1. Alimentary Canal. The non- imaginable proboscis (Fig. 183, 

 V, VI) leads directly into a true buccal cavity situated in the trunk 

 at the base of the foot (Fig. 182, /). In the interior of this buccal 

 cavity there is an azygos dorsal mandible and a ventral radula. The 

 radular sac is short, but its muscles and cartilages are powerful 

 and form a buccal mass of large size. The radula is short and 

 arcuate, with five teeth in each transverse row, the formula being 



