336 THE NATITBA.L HISTORY RETIEW. 



The Scolecida, wherein the digestive organs are thus reduced, 

 present, on the other hand, a remarkably advanced development in 

 their reproductive system. And though it is scarcely correct to speak 

 of any of these creatures as sterelminthous or parenchymatous worms, 

 still, what may be termed their general body-substance, or connective 

 system, often occupies much of the interior of the organism, encroach- 

 ing largely upon the perivisceral cavity. In both these points, relating, 

 it is true, rather to grade of complexity than to type of organi- 

 sation do the lower Helminths recall to mind the Infusoria. 



Two leading modifications of the reproductive system may be 

 indicated. Li the Cestoidea and Trematoda, which are hermaphro- 

 dite, the spermatozoa become developed into active filiform bodies. 

 The uterine and ovarian portions of the female genitalia always 

 remain anatomically distinct from each other, and there is added 

 a highly specialised yolk-forming apparatus, or ' vitellarium.* 



The Nematoidea and Gordiacea are, in most instances, unisexual, 

 and have their spermatic particles motionless, often rounded, but 

 never filiform. The ovary and uterus are merely dififerent regions 

 of the same continuous tube. 



The female apparatus of the Acanfhocephala may be considered 

 intermediate between that of the two types cited. Their sexes are 

 distinct, as in the Nematoidea and Gordiacea^ but the spermatozoa 

 resemble those of the Cestoidea and Trematoda, 



'In some Scolecida no nervous system has yet been discovered. 

 "Wlien present, its centres, like those of the lower neurosomatous 

 animals generally (^Ctenophora, Hotifera, Polyzoa, and Tunicata),* 

 consist of a single ganglion, or of two more or less approximated. 

 Much uncertainty still exists as to the true nature of the parts 

 called nervous among the Nematoidea and Gordiacea^ some authorities 

 doubting if these animals possess any nervous system whatever.f 



As to the various parts to which we have given the name of 

 * pseudo-vascular system (the true vascular system of other animals 

 being here, we need scarcely state, the general cavity of the body), 



* We here omit the Echinodermata, where the nervous apparatus presents fea- 

 tures altogether peculiar to this remarkable class. 



f It is, however, probable that future and more exact researches will detect 

 among the round worms a nervous apparatus vaiying much throughout the different 

 genera, but presenting among their higher forms a degree of complication greater 

 than that of the simpler type to which we have above referred. In short, the 

 nervous system of the Nematoidea will be found to differ from that of other 

 Annuloid animals, much in the same manner as the Brachiopoda, in this respect, 

 are distinguished from the lower MoUuscoids. 



