436 G. H. PARKER AND E. G. TITUS 



which reaches from the oral to the pedal disc and lies close 

 to the siphonoglyph. The directive mesenteries are always 

 sterile. 



Pairs of complete mesenteries, the non-directive, connect the 

 column wall with the undifferentiated oesophagus, that is, 

 with that portion which is not involved in the siphonoglyph. 

 They reproduce (fig. 2) in general the structure of the directives, 

 except that their longitudinal muscles are on their endocoel 

 faces, are flatter, and lie rather in their middle portions than 

 along their oesophageal borders. In some instances a non-direc- 

 tive mesentery may lack a marginal stoma or may have in place 

 of it two or three small openings as though the original stoma 

 had become partly divided (Carlgren, '93, p. 108). 



The pairs of incomplete mesenteries range in size from those 

 that are almost wide enough to be complete to those that are 

 mere ridges on the inner face of the column. In the widest 

 mesenteries (fig. 3) the highest degree of complexity is reached. 

 Such a mesentery carries on much of the oral portion of its 

 free edge a well differentiated mesenteric filament (fil.), below 

 which is a mesenteric convolution (cvl.). From the lower limits 

 of the convolution, an acontium (ak.) is given off. A series of 

 gonads (go.) extends along the edge from a point about opposite 

 the sphincter muscle to the region of the mesenteric convolution. 

 The longitudinal muscle is well defined and on the endocoel 

 face. There is naturally only one stoma, which corresponds to 

 the marginal stoma of the complete mesenterj^ (Hertwig, '79- 

 80, p. 523; Carlgren, '93, p. 108). The smaller incomplete 

 mesenteries differ from the one just described in the absence of 

 certain parts. As one passes from the larger mesenteries with 

 a full complement of parts to the smaller ones, the organs 

 are omitted in the following sequence. The first part to dis- 

 appear is the marginal stoma. Mesenteries that are so narrow 

 as to be without a marginal stoma still possess gonads, filaments, 

 convolutions, and acontia. The next organs to be lost are 

 the gonads; all smaller mesenteries are sterile. Then follows 

 the convolution. The simplest mesenteries are mere ridges 

 on the column wall carrying small mesenteric filaments and 



