31 



doubtless collected by ciliary currents induced in the first instance by the tentacles. It may fairly 

 be assumed that the food passes along the grooves situated on the niorphologically ventral or 

 internal sides of the arms down to the arm-bases, where they enter a passage Hmited in front 

 by the posterior walI of the proboscis and behind by the anterior collar-edge, which consists 

 more dorsally of the common base of the arms, and more ventrally of the opercular flap. The 

 operculum passes dorsally into the base of the last of the series of arms, to which it is related 

 in such a way that the food-groove of that arm becomes continuous with the internal layer of 

 epidermis of the collar-edge. 



It will be noticed that in the reconstruction the food-grooves are represented as dying 

 away before they actually reach the mouth. Masterm^n (98, 2, p. 507) has given an elaborate 

 account of the passage of these grooves, through the mouth, into the pharyngeal part of the 

 alimentary canal; and his results are discussed below, in dealing with the mouth. Although the 

 inner epidermis of the collar-fold is often a good deal wrinkled, I cannot agree with Masterman 

 that the food-erooves of the arms are continued as such into the mouth. Such sections as those 

 represented in PI. V, figs. 43, 45; PI. VI, figs. 65, 66 and PI. XII, figs. 145 — 147 shew that 

 the arm-base has a perfectly smooth surface in regions where, according to Masterman, grooves 

 should be present. The food-channel limited by the proboscis and operculum is an arrangement 

 so effective that it is hardly necessary to suppose that each food-groove must be continued 

 independently to the mouth. 



The lateral part of the operculum projects a considerable distance beyond the point 

 where the fifth arm joins it. This is indicated, not quite successfully, in fig. 22. and is also 

 apparent in fig. 25, in which the edge of the collar-fold is quite complete just ventrally to the 

 base of the fifth arm, while the lateral part of the operculum is cut by the (sagittal) section in 

 surh a way as to exposé its cavity. The same facts will be obvious from an inspection of the 

 sections, PI. V, figs. 50 — 53. 



This lateral projection of the operculum is so situated as to overhang the external 

 openings of the collar-canal (Fig. 25, c. c. c.) and gill-slit {£-. s. r.). I think it cannot be doubted 

 that the opercular lobe in question is functionally correlated with those apertures, and that it 

 serves to separate the food-current passing to the mouth from the presumably exhalant current 

 passing out of the gill-slit at least. The current through the collar-pore is probably sometimes 

 inwardly and sometimes outwardly directed. The proper play of the tentacles and arms, and 

 of the operculum itself, must depend on their being kept in a proper condition of turgidity. 

 In the Enteropneusta there is actual evidence (Spengel, 93, p. 475; cf also Ritter, 02) of 

 the importance of a turgid condition of the collar-cavity. Now the collar-canal itself appears 

 to have no sphincter muscles, and it seems to me probable that the lateral lobe of the operculum 

 is important as one of the means of closing the external orifice of the collar-canal during certain 

 conditions of the activity of that organ. Any excess of water which enters the pharynx is 

 probably evacuated by the gill-slits; and as I suggested on a former occasion (87, p. 44) it is 

 not impossible that this might have been the primitive function of the gill-slits of Chordata. 



The structure of the collar may be further elucidated by a consideration of the actual 

 sections on which the reconstructions, figs. 22, 25 (PI. III), are based. The obliquely sagittal 



