THE MARINE TURBELLARIA. 301 



the ductus itself. After entering the penis the two accessory ducts open into the ductus. The 

 penis is curved downwards and a little backwards ; it consists essentially of the end of the 

 ductus, protected by a tapering chitinous tube or stylet, with a narrow lumen, projecting into 

 the antrum masculinum. It can be protruded and withdrawn through the male gonopore by 

 the action of muscle-fibres, attached to the base of the chitinous stylet. 



The antrum consists of a small dorsal and a much larger ventral chamber separated from 

 each other by a circular fold of its wall. The roof of the dorsal chamber is of course formed 

 by the attachment of the basal part of the penis to the body. Its walls are lined with a 

 flattened non-ciliated (?) epithelium, through which the secretion of a large number of gland- 

 cells is being poured. These gland-cells lie beneath the epithelium, and may collectively be 

 called the prostate glands. The walls of the dorsal chamber form the ' penis-sheath.' The 

 ventral chamber of the prostate is lined with ciliated epithelium continuous with that of the 

 surface of the body. Its walls have a muscular coat, consisting of an inner circular and an 

 outer longitudinal layer of fibres. 



The accessory vesicles are both full of the coarsely granular secretion (fig. 20, s.) of the 

 gland-cells of the dorsal chamber of the antrum, and the same secretion is present in the 

 accessory ducts. This secretion cannot be the product of the accessory vesicles themselves, and 

 must reach them through their ducts. But it can only enter the ducts by passing from the 

 antrum through the external opening of the penis stylet, and travelling up the penis to the 

 opening of the accessory ducts into the ductus, and then turning down there. This Lang ob- 

 serves can come about in one of two ways. We may suppose that the secretion may be 

 driven up the accessor*- ducts by contraction of the walls of the antrum, the male aperture 

 and the ductus behind the openings of the accessory ducts being closed ; or the secretion may 

 be drawn into the accessory vesicles by a pumping action of the vesicles themselves. The latter 

 is perhaps the more probable method'. 



The species appears to be most closely related to von Plehn's P. nationalis from the coast 

 of Labrador [11]. 



15. Prosthiostomum cooperi sp. n. (PI. XIV. fig. 2). 



Two specimens, one semi-adult, the other immature, from Hulule, Male Atoll. 



The larger individual is 17 mm. long and about 5"5 mm. broad. 



Colour, milky white, margin of the body much folded. The adult has a series of fine 

 black circular markings, arranged roughly in two parallel rows running down the middle of 

 the back, from immediately behind the brain eyes, almost to the end of the body. 



The anterior margin is rounded, the posterior pointed. The anterior marginal eyes are 

 numerous and extend back along the margin considerably beyond the level of the brain eyes. 

 These latter are arranged in two long, almost parallel series, which diverge slightly from each 

 other at their ends. 



Body-wall much as in P. elegans, but the rhabdites are much more numerous. 



Genital apparatus. In both specimens the female organs are undeveloped. In the 

 semi-adult the male apparatus is mature, but unfortunately rather displaced owing to the 

 specimen having been damaged. 



' An examination of Lang's diagram of the genital organs account to be much more easily followed, see Lang [9] T. 30, 

 of Prosthiostomum sipliunculus will enable the foregoing fig. 20. 



