PHYLUM NEMATHELMINTHES 135 



anterior extremity of the body bearing hooks for grapphng into 

 the host tissues. This proboscis is frequently capable of inver- 

 sion into the anterior extremity of the body inside an organ 

 termed the proboscis receptacle or sheath. In some instances 

 the proboscis may be retracted within the anterior region of the 

 body without being inverted into the receptacle. 



Nuclear Constancy. — In all members of the family Neoechino- 

 rhynchidae, the individuals of each species are constructed on 

 exactly the same plan as far as the cellular elements are con- 

 cerned. Each organ and each tissue contains a definitely fixed 

 number of nuclei in the identical positions for every member of a 

 given species. In these forms, the size of the individual depends 

 wholly upon the size of the component cells, for all worms of 

 a given species are made up of the same number of cells. This is 

 a condition wholly different from that found in some other 

 animals, for in some species the size of the individual is depen- 

 dent upon the number of cells in its body. Typical arrange- 

 ment of nuclei in the organs of a single system is shown in 

 Fig. 71 B. 



The body wall is composed of an external layer of cuticula 

 which overlies a syncitial mass called the subcuticula. This 

 subcuticula forms by far the greatest bulk of the animal and is 

 provided with a few rounded giant nuclei, finely dendritic nuclei, 

 or numerous small nuclei. Two muscle layers, one with the 

 fibers directed longitudinally and the other with the fibers 

 directed circularly, mark the internal limit of the body wall and 

 bound the body cavity. 



At the anterior extremity of the body proper, two organs of 

 variable shape and of undetermined function, the lemnisci, 

 extend into the body cavity alongside of the proboscis recep- 

 tacle, apparently as continuations of the subcuticula. 



The central nervous system consists of a single ganglionic 

 mass within the proboscis receptacle. In the different genera, 

 this occupies a position varying from the posterior extremity of 

 the receptacle to a point near the anterior end of the receptacle. 

 Small branches are given off to the surrounding organs, and 

 usually a pair of structures called the retinacula pass through the 

 wall of the receptacle out to the body wall. 



Internal Organization. — Inversion of the proboscis is accom- 

 plished by a pair of invertor muscles which run from the tip of the 

 proboscis to the base of the receptacle through which they con- 



