THE LIFE-HISTOHY OF NUOULA DELPHINODONTA. 361 



muscle-fibres are always present. The epithelium covering 

 the outer surfaces of the suspensory membranes, those 

 surfaces that are turned away from the mid-line of the body, 

 is modified to form a portion of the hypobranchial glands. 



Each suspensory membrane bears two series of gill plates 

 that generally lie opposite to each other. Occasionally a 

 gill occurs in which there are more plates on one side than 

 on the other, but even in these cases the order is interrupted 

 oiily for a short distance. The number of plates differs with 

 the size of the individual, but about twenty pairs seems to 

 be common for well-grown specimens. Each plate is thin 

 and triangular, and is composed of epithelial walls, between 

 which there are loose connective tissue, large blood-spaces, 

 and the chitinous framework. The epithelial walls on the 

 edges of the plates that are directed away from the suspensory 

 membranes are thickened and covered with strong cilia. 

 This thickened ciliated epithelium extends between the plates 

 for a short distance, but most of the epithelium is quite thin 

 and destitute of cilia. The wall of each plate is continuous 

 with the wall of the plate that lies in front of it, with the 

 wall of the plate that lies behind it, and with the plate on the 

 other side of the gill that lies opposite to it. Near the border 

 furthest from the suspensory membrane, the opposing walls 

 of the two series of gill plates are separated so as to form a 

 large blood-space (fig. 53, hs.), that runs the whole length of 

 the gill. This space is continued as a narrow slit to the base 

 of the suspensory membrane. Thus the blood-space in the 

 suspensory membrane is in direct communication with the 

 blood-space of each plate, and in the gill the blood is free to 

 flow from one part to another. 



The chitinous framework consists of a bride-ed trouoh that 

 occupies the bottom and part of the sides of the blood-space 

 that lies between the two series of plates, and of two series 

 of side spouts that project into the plates on the two sides, 

 and lie in contact with the thickened epithelium. Although 

 the chitinous framework is arranged as a system of troughs 

 and spouts that, from their position, must be filled with 



VOL. 44, PART 3, NEW SERIES. A A 



