178 THE ANATOMY OF INVERTEBRATED ANIMALS. 



adult ; into which it eventually passes without any metamor- 

 phosis. 1 



Thus it appears that, in Asp>idog aster, the ovarium gives 

 rise to primary ova, which pass down the oviduct and become 

 fecundated, either by the spermatozoa conveyed by the inter- 

 nal vas deferens, or by those received by the vagina when 

 copulation with another individual, or, possibly, self-impreg- 

 nation, occurs ; that, next, the essential part of the process of 

 " yelk-division " takes place, the germinal spot dividing and 

 subdividing, and the primary ovum becoming in this way con- 

 verted into the spheroidal blastoderm ; that, contemporane- 

 ously, the blastoderm becomes invested by the accessory yelk- 

 granules poured in by the vitellarian duct, and by a shell ; 

 that the accessory yelk arranges itself into spheroidal masses, 

 which probably supply the blastoderm with the means of its 

 constant enlargement ; and that, finally, the accessory yelk 

 disappears, and the blastoderm becomes converted into the 

 embryo. 



The modifications exhibited by other Trematoda concern 

 the number of the suckers, of which there are usually several 

 in the ectoparasites, but not more than one in the endopara- 

 sites ; their support on a chitinous framework, or the addition 

 to them of spines or hooklets, similar to those of Cestoidea 

 or Acanthocephala : the bifurcation of the intestinal canal, 

 and the ramification of its branches, so that the forms of the 

 alimentary apparatus repeat the two extremes observed in 

 the aproctous Tarbellaria ; the existence of two nervous 

 ganglia with a single transverse commissure in many ; and 

 the occasional presence of sensory organs (eye-spots). The 

 non-contractile canals of some genera are destitute of cilia, 

 except at their inner terminations. 



The variations of the reproductive organs are rather of 

 position than of structure. Dioecious Trematodes are very 

 rare, the most important being the formidable IZilharzia, the 

 male of which is the larger and retains the female in a gynce- 

 cophore, or canal, which is formed by the infolding of the 

 margins of the concave side of the body. Bilharzia has 

 neither intromittent organ nor seminal pouch, and the history 

 of its development has not been traced beyond the escape of 



1 The substance of this account of the structure and development of Aspido- 

 gaster, with the illustrative figures, was published in 1856 in The Medical 

 Times and Gazette. M. E. Van Beneden has recently thrown much light on the 

 mode in which the ova of the Trematoda are formed and developed, in his 

 " Recherches sur la Composition et la Signification de l'CEuf." 



