12 JOHNS HOPKINS UNIVEESITY MOEPHOLOGICAL MONOGRAPHS. 



attached to the wall that separates the inhalent from the exhalent 

 siphon. The suspensory membranes are continued posteriorly as 

 ridges, beneath the posterior adductor muscle, and finally disappear. 



With the exception of a few of the plates at the extreme anterior 

 ends of the gills, which are sometimes much distorted and swollen, 

 all of the plates are alike in shape and structure. Laterally the plates 

 of each gill lie opposite each other. Generally in the length of each 

 gill one or more plates are interpolated on one side or the other, Fig. 

 20, and thus the plates are made to alternate for a short distance, 

 but they soon become opposite again. Kellogg's 5 opinion that the plates 

 of each gill are alternate and not opposite, is based on horizontal 

 sections taken below the upper longitudinal muscle, in which position 

 the inner edges of the plates are alternate. This seems to be due to 

 bending, caused, perhaps, by the pressure of blood in them. Horizontal 

 sections through the bases of the gills, as well as whole specimens, show 

 the plates to be opposite as a rule. 



The plates on the two sides of each gill are separated from each 

 other except below, where they are fused, either leaving a connecting 

 ridge, as frequently happens anteriorly, or a smooth intermediate 

 portion, as is generally the case further back. Each plate is also 

 joined to the suspensory membrane, Fig. 19, gs., above, and to two 

 longitudinal muscles, llm. and ulm., that are continuous the length 

 of the gill. Each plate is hollow and may be likened to a loosely 

 folded sheet of paper, leaving a space between, which space is traversed 

 by a few fibres of connective tissue. The unshaded line, Fig. 19, 

 represents the wall that has been cut in separating the plate from 

 the plate lying in front of it. The opposite wall of the plate is 

 continuous with the wall of the plate lying behind it. Along each side 

 of the ventral portion of each plate, lying just inside and closely applied 

 to the wall, is a chitinous rod or plate Fig. 19, cr. These rods are 

 quite heavy near the median line of the gill, become thinner laterally, 

 and finally disappear. Furthermore the chitinous rod on each side 

 of each plate is continuous with the chitinous rod of the opposing 

 side of the plate adjoining. Each rod might then be described as 

 hairpin-shaped, with two prongs, each of which extends along the 

 inner side of the wall of one side of one plate. Each plate receives one 

 prong from each of two hairpin-shaped rods, Fig. 20, cr. 



Many of the muscle fibres of the suspensory membrane are 

 continued down along the inner surface of the anterior wall of each 



