66 



INVERTEBRATE MORPHOLOGY. 



the ectoderm consisting of a number of ciliated cells arranged 

 in a single layer and enclosing a mass of germ-cells which 

 correspond to the central cell of Dicyema. Between the germ- 

 cells and the ectoderm fine nucleated fibres occur which are 

 presumably muscular. 



Three forms of individual are known to occur in the genus 

 Rhopcdura, one being a male, and the other two females. 



Fig. 33. — Rliopalura Oiardii (after Jnuu). 

 A, male ; B, round female ; C, flat, female. 



The male (Fig. 32, A) is about half the size of the females, 

 which measure about 0.25 mm. in length, and presents a met- 

 americ arrangement of the ectoderm which does not extend to 

 the internal cells. The cells of the anterior segment have 

 their cilia directed anteriorly, and are succeeded by a segment 

 consisting of several rows of small non-ciliated cells each 

 containing a refractive body, and behind this there follow three 

 or four segments formed of cells provided with cilia directed 

 backwards. One of the female forms (Fig. 32, B) is elon- 

 gated, and is segmented like the male except that the segments 

 are more numerous and the second non-ciliated segment con- 

 sists of a single row of cells destitute of refractive bodies. 



