6 Papers from the Marine Biological Laboratory at Tortugas. 



pore being in the dorsal surface a short distance from the lateral margin 

 of the body. 



The nervous system is also of the usual type, the brain lying dorsal 

 to and just in front of the pharynx (fig. i, h.) and the anterior and pos- 

 terior paired nerves extending from its lateral ends to the extremities of 

 the body (fig. i,a. n., I. n.). The main posterior nerves run lateral to the 

 massive yolk-glands and along the margin of the body. No ocelli are 

 present. 



The genital organs are similar in arrangement to those of M. ijimoB. 

 The single testis (figs, i and 8, /.) is a spherical structure 0.115 mm. in 

 diameter, which lies just back of the middle of the body at the left of the 

 median line. In M. ijimcB three testes are present, while in M. myliohatis 

 no one has yet seen any of the male genital organs. The vas deferens (figs. 

 I and 8, v. d.) is a narrow tube with rather thick, structureless, highly 

 refractive walls; it arises from the anterior surface of the testis and 

 passes forward at the left of the median line to the vaginal pore (figs, i 

 and 8, v. p.), which is situated in the ventral surface on the left side of 

 the body near the hinder end of the pharynx. No penis, cirrus, or vesicula 

 seminalis is present, the vas deferens having exactly the same structure 

 and diameter, from one end of it to the other. I have also been unable 

 to detect any trace of specialization in the parenchyma surrounding the 

 anterior end of the vas deferens, such as is present in M. ijinics and 

 forms the so-called connective-tissue penis in that species. 



The ovary (figs, i and 8, o) is made up of two distinct portions, one 

 of which, the formative portion, is spherical in shape and about the same 

 size as the testis, and lies immediately in front and at the side of it at 

 the right of the median line. The other portion is cylindrical in shape 

 and extends from the spherical portion to the right side of the body, 

 where it forms a single loop around the right intestinal trunk, tapering 

 in size as it proceeds until it becomes so narrow that it contains but a 

 single row of elongated ova. It thus runs to the left side of the body, 

 where it passes, without further decrease in size, into the oviduct, which 

 is a very short tube (fig. 8, ov.), which joins the posterior surface of the 

 ootype. The ova filling the ovary are of large size, those in the cylindrical 

 portion being about 0.025 mm. long and 0.013 mm. thick. 



The ootype (figs, i and 8, oot.) is a large spherical sac, about 0.09 mm. 

 in diameter and with thick walls, which is situated just in front of the 

 ovary on the left side and near the dorsal surface of the body. The walls 

 are composed of a high pavement-epithelium of deeply staining, glandu- 

 lar cells, which forms the interior surface of the organ, and a layer of 

 rather delicate muscle-fibers lying back of it. Grouped at the antero- 

 medial side of the ootype, where the uterus leaves it, are the large pear- 

 shaped cells which form the shell-gland. The ootype occasionally con- 

 tains a single large oval egg (fig. 8, e.), which about half fills it and has a 

 length of about 0.045 mm. and a thickness of 0.03 mm. Projecting 

 from the hinder end of the egg, as it lies in the ootype, is a short, straight 

 chitinous process about 0.02 mm. long. It seems certain that the chitin- 



