182 AN INTRODUCTION TO ZOOLOGY 



appear to play an active part in this process, by collecting up the 

 various granules and conveying them to the nephrostome, where they 

 yield them up again. It is considered by some authorities that this 

 excretory function on the part of the peritoneal cells is a primitive 

 one, and was the sole one before the evolution of the special and 

 complex segmental organs, the nephridia. 



The muscular system in the worm is a very simple one, 

 consisting in the main of two double sheaths of muscle fibres. The 

 first is the somatic or body wall musculature, and it consists of a 

 series of muscle fibres arranged circularly all along the animal 

 underneath the epidermis, save where it is interrupted interso- 

 mitically at the grooves, and a much thicker layer of longitudinal 

 muscles. The latter are bounded internally by the lining of the 

 body cavity, the peritoneum, and are arranged in bands. Two 

 dorso-lateral bands extend from the mid-dorsal line, where they are 

 just separated from one another in the line of the dorsal pores, to 

 the line of the lateral setae, whose enveloping sacs again form a break. 

 The two latero- ventral bands occupy the spaces between the lines 

 of lateral and ventro-lateral setae, and, lastly, a ventral band 

 occupies the mid-ventral line. The tiny longitudinal muscle fibres 

 are arranged on each side of a large number of radially-running 

 partitions, and exhibit in transverse section a very characteristic 

 feather-like appearance. The second sheath is the splanchnic or 

 gut wall musculature, and it consists of an inner very thin layer of 

 circularly arranged fibres lying beneath the entoderm and an outer 

 even thinner layer of longitudinal fibres. These two layers are 

 considerably developed in the pharynx and gizzard. They are 

 bounded by the splanchnic or gut layer of peritoneum, whose cells 

 are highly modified in the intestinal region to form the chloragogen 

 cells. In addition to these sheaths there are the special muscle 

 strands already mentioned as radiating from the pharynx. All the 

 muscles seem to be composed of cellular non-striated fibres. 



The normal method of reproduction takes place sexually 

 by the production of gametes, but if a worm is cut up into a number 

 of pieces, each, as in Hydra, possesses the power of regeneration, 

 enabling it to regrow the lost parts. Thus the agriculturist who 

 viciously cuts a worm in halves with his spade is doing his land more 

 good than he knows, not by removing a worm, but by adding to 

 their numbers. Lwnhricus is monoecious or hermaphrodite, the 

 two essential sexual organs, ovaries and testes, occurring in the 



