478 



ZOOLOGY 



SECT. 



a supporting rod ; in the intervals between these there are in 

 Myzostoma four pairs of small " suckers " ; and round the margin 

 are a series of ten or more pairs of cirri provided terminally with 

 motionless sensory cilia, and with a ventral groove lined by adhesive 

 cells. The mouth, usually situated at the anterior extremity, leads 

 into a muscular pharynx (Fig. 392, ph.) capable of being protruded 

 as a proboscis ; from this a narrow oesophagus leads to the stomach, 

 which gives off a number of branched lateral diverticula (da.). A 

 short cloaca (Ho.) leading from the stomach opens on the exterior, 



in most cases at the 

 posterior end of the 

 body, sometimes on the 

 dorsal surface. There 

 is no distinct ccelome, 

 the space between the 

 alimentary canal and 

 the body-wall being 

 filled by connective- 

 tissue (parenchyma), 

 leaving only the cavi- 

 ties in which the sexual 

 elements are lodged. 

 Bundles of dorso- 

 ventral muscular fibres 

 form imperfect trans- 

 verse septa, as in some 

 Platyhelminthes. 



There is no blood- 

 vascular system, and 

 specialised organs of 

 respiration are likewise 

 wanting. There is a 

 single pair of nephridia 

 with funnel-shaped in- 

 ternal apertures and 

 with external openings 

 either into the cloaca 

 or on the surface. The 

 nervous system comprises a large stellate ganglion situated ventrally, 

 probably representing a number of fused ganglia, and giving off a 

 number of nerves ; and two nerve-rings, one round the 

 oesophagus, the other round the pharynx, the two rings being 

 connected together by a series of longitudinal nerves. The 

 cesophageal ring presents a very obscure dorsal thickening, which 

 is the only representative of a cerebral ganglion. 



Most of the Myzostomida are hermaphrodite. There is a pair of 

 ovaries formed by the proliferation of the layer of (coelomic) 

 epithelium covering the stomach ; and in the sexually mature 





FIG. 391. Myzostoma. I X, cirri; m. mouth 

 p. parapodia ; s. suckers. (After von Graff.) 



