THE PROSOMATIC SEGMENTS OF AMMOCCETES 30 1 



belongs to the same segment as the lower lip is the fact that in 

 addition to the tubular muscles a single ordinary striated muscle is 

 found in the velum which, like the muscles of the lower lip, is 

 innervated by this same mandibular nerve. 



This muscle is attached laterally to the muco-cartilage of the 

 metastomal bar (s& 2 ) at its junction with the muco-cartilage of the 

 lower lip, and spreads out into a number of strands which are 

 attached at intervals along the whole length of the free anterior 

 edge of the velum. It is the only non-tubular muscle belonging 

 to the velum, and by its contraction it draws the anterior portions 

 of the velar folds apart from each other, and so opens the slit 

 between them, through which the food and mud must pass. Clearly 

 from its position it does not belong to the original tergo-coxal group 

 of muscles as do those of the lower lip ; it must have been one of the 

 intrinsic muscles of the metastoma itself. 



This anterior portion of the velar folds affords yet another 

 striking hint of the correctness of my comparison of the lower lip 

 segment of Ammoccetes with the chilaria of Limulus or the metas- 

 toma of Eurypterus ; for the most dorsal anterior portion, which at 

 its attachment possesses a wedge of muco-cartilage, forms a separate, 

 well-defined, rounded basal projection marked Ser.ia Fig. 115, and B 

 in the accompanying Eig. 120. This is that part of the velar folds 

 which comes together in the middle line and closes the entrance into 

 the respiratory chamber. The epithelial surface here is most striking 

 and suggestive, for it is markedly serrated, being covered wuth a 

 large number of closely-set projections or serree. The serration of 

 the surface here is of so marked a character that Langerhans con- 

 sidered this part of the velar folds to act as a masticating organ, 

 grinding and rasping the food and mud which passed through the 

 narrow slit. In fact, Langerhans supposed that this portion of the 

 velum acted in a manner closely resembling the action of the gnatho- 

 bases of the prosomatic appendages in Limulus or the Eurypteridae. 



This suggestion of Langerhans' is surely most significant, con- 

 sidering that this somewhat separate portion of the velum, to which 

 he assigns such a function, is in the very place where the gnathite 

 portion of the metastomal appendages would have been situated if it 

 were true that the lower lip and anterior portion of the velum of 

 Ammoccetes were derived from the metastoma. 



In addition to this marked serrated edge the whole surface of 



