CCELELMINTHA. 157 



ovaria, to aggregations of spermatozoids provided with tails and subse- 

 quently isolated. At length the body of the male Nemertean appears to 

 be as completely stuffed with spermatozoids as that of the female is with 

 eggs, and they are ultimately expelled, in prodigious numbers, into the 

 surrounding water. 



(419.) The CraLELMiNTHA, or cavitary intestinal worms of Cuvier, 

 evidently present a much higher type of structure than any of the pre- 

 ceding. The Ascaris lumbricoides, as its name imports, so strongly 

 resembles some of the Annelida in its external configuration, that the 

 zoologist who should confine his attention to outward form alone, might 

 be tempted to imagine the affinities connecting them much stronger than 

 a comparison of their anatomical relations would sanction. This Entozoon 

 is found in the intestines of many animals, and is endowed with some 

 considerable capability of locomotion, adapted to the circumstances under 

 which it lives ; for in this case the worm, instead of being closely im- 

 prisoned in a circumscribed space, may traverse the entire length of the 

 intestines in search of a convenient locality and suitable food. 



(420.) In accordance with such an enlarged sphere of existence, we 

 observe muscular fibre distinctly recognizable in the tissues that com- 

 pose the walls of the body not as yet, indeed, exhibiting the complete 

 characteristics of muscle as it is found in higher animals, but arranged 

 in bundles of contractile filaments, running in determined directions, 

 and thus capable of acting with greater energy and effect in producing 

 a variety of movements. 



(421.) In this rudimentary state, the nmscular fibre does not possess 

 the density and firmness which it acquires when completely developed ; 

 it has, when seen under the microscope, a soft, gelatinous appearance, 

 apparently resulting from a deficiency of fibrin in its composition ; the 

 transverse striae, usually regarded as characteristic of the muscular 

 tissue of the more perfect animals, are not yet distinguishable ; and the 

 individual threads are short, passing over a very small space before they 

 terminate. On examining the arrangement of these fasciculi, they are 

 seen to be disposed in two layers, in each of which they assume a dif- 

 ferent course ; thus, in the outer layer they are principally arranged in 

 a longitudinal direction, while the inner stratum of fibres is placed trans- 

 versely, affecting a spiral course, so as to encircle the viscera. Erom 

 this simple structure various movements result : by the action of the 

 longitudinal fasciculi the whole body is shortened ; by the contractions 

 of the spiral layer an opposite effect is produced ; or by the exertion of 

 circumscribed portions of the muscular integument, lateral flexions of 

 the body are effected in any given direction. These motions in the 

 living worm are vigorous, and easily excited by stimuli ; they are there- 

 fore abundantly sufficient for the purpose of progression in such situ- 

 ations as those in which the creature lives, and enable it to change its 

 place in the intestines with facility. 



