MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 1 19 



embryonic gelatinous tissue. Probably better preserved material would show more structure 

 than I have been able to discover. When the skin has been removed from the labial lobes their 

 muscles come into view. 



The muscle fibres of the superior labial lobes pass into the tissues of the cephalic sheath 

 without forming any distinct muscles. But in regard to the inferior labial lobe the case is very 

 different. 



A longitudinal median muscle, the dorsal median retractor (Fig. 26,5), extends over the 

 dorsal face of the lobe from just back of the lamellated organ to the base of the lobe, where it 

 enters the ventral portion of the cephalic sheath immedately above the funnel. A similar, but 

 much smaller, longitudinal muscle follows a similar median course upon the ventral face of 

 the lobe. 



There remain three pairs of muscles arranged symmetrically at the sides of the lobe. 1 and 

 1' (Fig. 26) extend out and back from the sides of the lobe to insertions in the lateral regions of 

 the cephalic sheath immediately below the bases of the superior labial lobes. These appear to be 

 the muscles used in raising the lobe and its tentacles. 



2 and 2' (Fig. 26) originate near the base of the lobe and pass outward from the median line 

 over the base. The outer ends of these muscles are expanded and flattened, and are inserted 

 over the inner faces of the superior labial lobes. This arrangement evidently serves for the 

 approximation of the superior and inferior lobes and their tentacles. The posterior portions of 

 the inner ends of 2 and 2' separate from the remainder of the muscles and unite with each other 

 above the median muscle 5. 



The muscles 3, 4, and 3', 4', (Fig. 26), the lateral retractor muscles, have their origins in 

 the posterior region of the lobe beneath the inner parts of the muscles 2 and 2'. They pass 

 beneath these latter backward and slightly toward the median line. Arising as single muscles, 

 they separate into two distinct portions. The smaller outer portions pass into the base of the 

 buccal mass behind the inferior buccal retractors, as is shown for similar muscles in the figure of 

 Van deb Hoeven's organ. ( Fig. 20. ) The larger inner portions pass back to insertions upon the 

 median processes of the cartilage. 



The base of the Jobe is thick and muscular and its tissues extend directly into the ventral 

 portion of the cephalic sheath. 



The inferior labial lobes are supplied with blood by a branch from each tentacular artery. 

 (Text-tig. 10, pp. 182, 3.) 



A large nerve leaves each pedal ganglion near the infundibular nerve (Fig. 41, 7), which runs 

 forward and enters the side of the inferior labial lobe. Within the lobe it enlarges into a gan- 

 glion, from which the separate nerves of the tentacles and the lamella? arise. (Fig. 41, 35.) 



It must be noted that while the infei'ior labial lobe is bilaterally symmetrical it is not paired 

 as are the superior labial lobes. Young specimens seem to indicate that it is unpaired in its 

 origin. 



We have still to consider what the relation is between the lamella? upon the median part of 

 the lobe and the tentacles upon either side of the lamella?. Can the two sorts of structures be 

 homologous, or must they be considered as developed separately — one for adhesion and the other 

 for smelling, tasting, or some other function? 



It has already been noted that as we approach the lamella? the inner cirri of each side, i. e., those 

 nearest the lamella?, rapidly decrease in length. Hand in hand with the decrease in length of the 

 cirri goes a decrease in the depth of their sheaths. Moreover, we often find that the tentacle 

 next to the lamella 1 is a small, scarcely noticeable papilla. In some cases such a papilla is partly or 

 wholly surrounded by a shallow groove. Further development of papilla and groove would lead 

 to the formation of a cirrus and a sheath, and tentacles sometimes actually illustrate steps in this 

 process. The minute papilla?, annularly grooved, are supplied with a nerve similar in all respects 

 but length to the nerve of fully developed cirri. 



On the other hand, the outer lamella" are sometimes small and not much different from the 

 rudimentary tentacles just described. So, if we consider shape alone, it is not at all difficult 

 to imagine that lamella 1 and tentacles are but differently developed individuals of a single series. 



