THE CCELOMATA INVERTEBRATA 197 



waving movements, which give to the group as a whole a curious 

 flickering movement, recalling a candle flame in a slight draught, and 

 hence they are termed " flame cells." They are the characteristic 

 excretory organs of the class of flat worms to which Tcenia belongs, 

 and the lumen of the tubules at the end of which they occur is 

 intracellular. 



A transverse section shows that the proglottid is solid and covered 

 by a cuticle, which is secreted by a layer of modified sub-cuticular 

 cells. These represent a transformed epidermis, which has come to 

 be partly embedded in the underlying tissue. The whole of the 

 interior not occupied by definite organs is filled up with very cha- 

 racteristic cellular padding tissue, the parenchyma. This is a meso- 

 dermal product, and although a large coelom like that in Lumbricus 

 is never developed, we still classify them as coelomates, mainly on 

 account of the development of the mesoderm. A few scattered 

 cells in the parenchyma secrete small calcareous particles, which are 

 of a spherical or oval shape. The muscular system is only feebly 

 developed, as there is but little need for movement, and it consists 

 of a thin dermal layer and a deeper layer made up of thin sheets 

 running in transverse and oblique directions, some forming strands 

 passing through the parenchyma from side to side. 



As has been pointed out, the proglottids just behind the neck 

 are immature and have at first no reproductive organs. These soon 

 start to develop, the male organs appearing first, followed shcrtly 

 after by the female structures. They are all found fully developed 

 in each mature proglottid, which is therefore hermaphrodite. 



The male gonads, the testes, consist of a number of globular 

 follicles scattered generally throughout the parenchyma, save in the 

 middle of the posterior end of the proglottid. They all give off 

 delicate tubes, the vasa efferentia, which unite to form larger and 

 larger trunks all ultimately leading to one tube, the vas deferens, 

 about the middle of the segment. This passes transversely to one 

 side or the other, and opens with the female aperture into a small 

 common cavity, the genital sinus, on the lateral margin of the pro- 

 glottid. The sinus opens by an elongated slit, the genital pore. The 

 end of the vas deferens is contained in a muscular sac, and is modified 

 to form a protrusible organ sometimes called the penis, but better 

 termed the cirrus. 



The female gonads, the ovaries, are two conspicuous plate-like 

 structures lying near the posterior end of the proglottid, and joined 

 by a transversely running canal. From the middle of this comes off 

 the oviduct, which runs backwards towards the end of the segment, 

 receiving shortly after its origin, the vagina. Near the posterior 

 border the oviduct turns sharply round, receives a duct from the 



