NEMERTEA. 



269 



depending merely on the size of the organ. The flame-cell knobs 

 of JSTemertines sometimes project into the blood-vessels, but do not 

 open into them. In the forms in which flame-cells have not been 

 observed, the excretory canals are said to open into the lateral blood- 

 vessels. This will probably turn out to be an error. The animals 

 are generally dioecious, and the gonads quite simple, consisting of 

 paired sacs regularly placed between the gut caeca, or in some cases 

 more numerous than the caeca and irregularly arranged. They open 

 by simple openings on the dorso-lateral surface of the body. 



Some genera are viviparous (Prosorochmus Claparedii, Tetrastemma 

 obscurum), but most of them lay eggs in albuminous strings. The 

 development is usually direct, but in many Heteronemertini the young 

 are hatched as helmet-shaped, free-swimming larvae (Fig. 223).. 

 known as Pilidium. 



Fio. 223. Pilidiinn- (after E. Metschnikoff). a, free-swimming larva. 6, later stage, helmet- 

 shaped. 7-,', E' the two pairs of ectoilermal iuvaginations ; D alimentary canal. 



The Pilidium was formerly described as a species of a supposed 

 independent genus. The enteron is formed by the invagination of 

 the wall of a hollow blastosphere ; the blastopore forms the larval 

 mouth ; at the aboral pole a long flagellum is developed, and a 

 broad lobe grows out on each side of the mouth, on to the edges 

 of which the circumoral ciliated band extends. By an elaborate 

 metamorphosis this larva turns into the young worm. 



Asexual reproduction is unknown, but the power of repair is great. 

 In some cases the body readily breaks up into pieces when touched, 

 and the pieces have the power of developing the whole. 



The Nemertines live principally in the sea, under stones in the 



