24 ORGANS OF SUPPORT, 



seen at 0, the abdominal nerves are seen as a single line 

 running along the middle of the ventral surface, and separat- 

 ing to encompass the vulva at b, and the anal opening is 

 seen near the posterior termination of the body at d. In 

 the jointed teenies and bothriocephali, this covering is soft 

 and thin, these being almost aggregates of simpler animals, 

 and it is still more thin and dilatible in the hydatids and 

 caenuri. This exterior covering often presents an irregular 

 transversely-corrugated appearance during the contracted 

 state of the long filiform antozoa, which arises from the still 

 irregular attachment of the interior interrupted longitudinal 

 muscular fibres ; and these irregular transverse corrugations 

 present us with the first condition of those joints and rings 

 which become so regular and distinct in higher articulated 

 classes. This exterior soft covering of the entozoa presents 

 us also with the lowest form of that cyclo-vertebral element, 

 which forms by its consolidation and repetition, a series of 

 calcified rings or segments around the exterior of the trunk 

 in higher entomoid articulata, and the solid bodies of the 

 vertebrae in the red-blooded classes. 



In most of the inferior orders of entozoa there are numer- 

 rous dense conical recurved hollow, and sharp pointed spines 

 which are sometimes disposed as teeth around the mouth, 

 and sometimes are found covering a great portion of the 

 anterior part of the body, giving it the appearance of a file 

 to abrade the surface on which they are to feed, or through 

 which they have to force their way. These hollow spines 

 are thus organs of progressive motion, like the cirrhi and 

 setse of annelides. They are strongest and most numerous 

 in the acanthocephalous species, where they cover the whole 

 of the retractile proboscis, and sometimes, as in the echino- 

 rhynchus hystrix, nearly the whole of the anterior surface 

 of the body. There is thus a transition from these teeth-like 

 organs in the interior of the mouth, and those covering the 

 surface of the body in these soft worms which move through 

 a fleshy resisting medium everywhere in contact with them, 

 to the lateral spines of the annelides which can be moved 

 with more freedom and more precision through a thin aquatic or 

 aerial medium, as in the aquatic annelides and the earth-worms. 



The exterior covering is more dense in its texture, and 

 consequently more articulated in its appearance in those 



