MOLLUSCA. 



arranged in a circle, project ; they serve for creeping and swimming, 

 as well as for the capture of prey, and usually bear rows of suckers 

 on their oral surface. In many forms (Octopoda) the basal parts of 

 the arms are united by a membrane which forms a kind of funnel in 

 front of the mouth, the cavity of which is contracted and dilated 

 in movement (not to be confounded with the pedal funnel, fig. 

 530 T). In others two lobe-like cutaneous appendages, the so- 

 called fins, serve for swimming (fig. 531) : these forms (Decapoda) 



possess in addition to the eight 

 arms a pair of very long ten- 

 tacles (fig. 531). 



In Nautilus, the single 

 living representative of the 

 Tetrabranchiata, there is 

 found in place of the eight 

 arms a crown of very numer- 

 ous tentacles. These, how- 

 ever, according to the view 

 of Valenciennes, appear to 

 correspond morphologically 

 to suckers; in fact similar 

 filaments are found on the 

 arms of Cirroteuthis, as pro- 

 longations of the cylindrical 

 nucleus of the suckers. The 

 true arms of Nautilus are 

 very short and rudimentary, 

 orming fold-like lobes at the 

 base of the tentacles. 



The funnel is placed on the 

 ventral (posterior) side and 

 projects from the broad opening of the mantle cavity, which can be 

 closed laterally by suckers. It has the form of a cylindrical tube, 

 narrowed at the front (free) end, and in Nautilus is open along the 

 under surface. Its broad base is placed in the mantle cavity, and it 

 serves to conduct away to the exterior from the latter the respiratory 

 water which has entered by the general mantle-opening, and with it 

 the excrementitious and generative products. At the same time, act- 

 ing in conjunction with the powerful mantle musculature, it serves as 

 an organ of locomotion. The respiratory water is violently driven 

 through the funnel by the contraction of the mantle, the general 



FIG. 531. Loligo vulgaris (after Verany). 



