ON THE ANATOMY OF HISTEIOBDELLA HOMARI. 291 



were about eighty, was first outlined on paper with the aid of 

 a camera lucida, and the nervous system carefully marked 

 in. Each of these drawings was then measured transversely 

 from side to side, and the measurements plotted out on milli- 

 metre paper, allowance being made for the magnification 

 between sections. The nervous system was also measured, 

 and likewise put in, all the distances being doubled to give 

 an axial line. The ends of the plotted points were then joined 

 up, and an outline of the external form and the nervous 

 system obtained. The figures Were then reduced to their 

 present size, and at the same time transferred to ordinary 

 drawing-paper by means of an eidograph. In the sagittal 

 section shown ia fig. 15 the dorso-ventral diameter was taken 

 instead of the transverse. By this means the relationship of 

 the ganglia to the segments can be accurately determined in 

 a way that would be impossible with the ordinary recon- 

 struction methods (figs. 15, 21, 28). 



. 2. Historical Review and General Eemarks on Habits, 



ETC. 



Histriobdella was discovered and briefly described by 

 J. P. van Beneden (1) in 1853. He found it as a parasite on 

 the eggs of some lobsters obtained from Ostend. He con- 

 sidered it a larval Serpulid, and placed it among the Poly- 

 chseta3. Subsequently, in 1858, he (2) pointed out that it was 

 an adult form. From its peculiar structure he remarked that 

 it could not be easily classed with any known group of animals, 

 although some of its features he thought were such as to 

 place it among the leeches. He gave a more or less detailed 

 description of both sexes, and figured the eggs and immature 

 young. 



To Foettinger (8) we owe the most extensive account of this 

 animal. He describes the nervous system, nephridia, repro- 

 ductive organs, and, in fact, was the first to give a detailed 

 account of its anatomy based on sections. He supported the 

 conclusions of Edouard van Beneden that it was an Archi- 

 annelid, placing it near Polygordius, but separate from it. 



