SPINNERET (S. s. 447) 145 



Action of Spinneret. The structure of the spinneret is very similar 

 to that of Mononchulus, a related genus whose unusually large spinneret 

 has proven especially favorable for detailed study, and may be described 

 as follows. The three somewhat elongated ampullae at the ends of the 

 caudal ducts empty into a single sac located in the spinneret. From the 

 front wall of this sac there projects backward into its cavity an elongated 

 element whose free conical distal extremity fits into the outlet of the 

 spinneret, where it acts as a plug or valve. Apparently, this valve must 

 arise through an invagination of the sac during development. The plug 

 is withdrawn by muscles attached to it and to the dorsal body-wall near 

 the end of the tail. A contraction of these muscles elongates the sac con- 

 taining the caudal secretion and draws the plug away from the aperture 

 of the spinneret, thus allowing the secretion to flow out through the termi- 

 nal pore. The plug appears to be returned to its position and held there ' 

 either by the pressure of the secretions in the sac, or by the general in- 

 ternal body pressure exerted on it through the sac. Figure 5 shows the 

 similar spinneret of a mononch. 



Use of Spinneret. In any genus consisting of many species most of 

 which have caudal glands, aberrant species usually occur in which these 

 organs are absent. This is true of Mononchus. A spinneret would ap- 

 pear to be a particularly useful organ to a predaceous nema, permitting 

 it at will instantly to cement itself to a relatively fixed object, and thus 

 more easily master an active victim ; nevertheless, about one-fourth of the 

 mononchs have no trace of such an organ. 



The lateral fields are always comparatively well-developed, and are 

 usually one-third to one-fourth as wide as the body. The cells composing 

 them often contain spherical granules not differing greatly in size from 

 those of the intestinal cells. No cuticular pores have ever been seen con- 

 necting elements of the lateral fields with the exterior. 



Renette 



Nothing is known about the structure of the renette in fact, it is not 

 yet definitely known to exist in mononchs. In most species there is a 

 ventral pore immediately behind the nerve-ring, which bears a close re- 

 semblance to the excretory pore of nemas possessing a well-developed 

 renette; but no internal structures have as yet been found to connect 

 with this pore. 



Central Nervous System 



The nerve-ring surrounds the oesophagus somewhat squarely near the 

 front end of the middle third of the neck. It is usually well developed 

 and easily seen a distinct refractive collar with groups of nerve cells 

 both in front of it and behind it. Otherwise than this very little is known 

 about the central nervous system of Mononchus. 



