284 F. F. LAIDLAW. 



Genital apparatus. The characters are shown somewhat diagrammatically in PI. XV. 

 fig. 11. In the female system the accessory vesicle (a. v.) of the vagina is thread-like 

 and extends back as far as the level of the female aperture ( $ ). After receiving on its ventral 

 side the common duct from the two uteri (c. d.), the vagus (va.) follows a winding course 

 through the enormous shell glands (s. g.), and runs into a large bursa copulatrix {b. c), whose 

 lumen has many folded walls, appearing in part at any rate to be glandular. As in other 

 typical members of the genus the terminal parts of the male apparatus are enclosed in an 

 outer muscular sheath (o. s.), in which lie the penis and the prostate (pr.) as well as the 

 duct. A remarkable feature of the penis, and one that separates this species sharply from 

 other known Planocera, is the presence at the end of that organ of six large chitinous 

 hooks (c. h.) and at its upper end of a chitinous ring or collar (c. r.). The lumen of the 

 penis is lined by a very large number of small chitinous spines (c. s.). These vary from "05 

 to '07 mm. in length. The space between the penis and outer sheath is occupied by a very 

 loose parenchymatous tissue. 



Female organs. At its hinder end the accessory vesicle (a. v.) is difficult to trace 

 owing to its extremely small size. The lining cells are not ciliated. As it passes forward 

 it increases in size and a very few circular muscles appear about it. After receiving the 

 short common duct (c. d.) from the uteri, which are also lined with non-ciliated epithelium, 

 the vagina {va.), as it must now be called, continues to run forwards for some little 

 distance. It then turns first ventralwards and then backwards. As it turns back it 

 widens out so that its lumen is wide from side to side but narrow dorso-ventrally. In this 

 part of its course it receives the secretion of the relatively enormous shell-glands (s. ^r.), 

 which have a characteristic granular appearance and stain very lightly. The vagina continues 

 to run backwards, passing right through the shell-glands until it lies right under the front 

 end of the bursa. Here its lumen becomes circular in section. It now makes a Z-shaped 

 turn ujjwards, and runs into the hinder end of the large bursa copulatrix {b. c). As the 

 vagina passes into the bursa its lining epithelium undergoes a marked change. Up to this 

 point it consists of a fairly well-defined, cubical epithelium with deeply staining nuclei, 

 apparently non-ciliated. As it passes into the bursa it becomes irregular and much folded; 

 the protoplasm stains more deeply, and the nuclei can with difficulty be distinguished. 

 From its proximal, anterior end the epithelium lining the bursa {g. I.) retains these charac- 

 ters almost to the gonopore, in the neighbourhood of which it merges into a columnar 

 non-ciliated epithelium, continuous with the epithelium of the surface. The epithelium of 

 the bursa is secreting very actively a remarkable ' glairy ' substance, which occupies nearly 

 the whole lumen of the bursa. Outside this epithelium the bursa is built up of a thick 

 muscle layer of circular fibres. The folding of the wall lining the lumen of the bursa is 

 no doubt, as Lang suggests, to permit of the distension of the organ. The secretory character 

 of its wall is, so far as is known, confined to this species, and without homologue in the 

 bursa of any Polyclad I am acquainted with. This peculiarity combined with the very remark- 

 able nature of the penis indicates that this species is one of the most specialized in the 

 ■whole order. 



Male apparatus. The gonopore opens into an antrum masculinum whose walls closely 

 resemble those figured by von Graff for his PL simrotlii [6]. Little spherical masses of granular 

 secretion given off from the cells lining the antrum lie scattered in its cavity. The outer 

 sheath (PI. XV. fig. 12, o. s.) is composed of two layers of muscle fibres, an outer longitudinal 



