112 THE ORIGIN OF PAEASITES. 



cavity, which in both cases contains a well developed male or 

 female sexual apparatus, whose differences, although apparent even 

 on a superficial view, are scarcely more marked than those found 

 in the structure of the same apparatus in the Chsetopoda or the 

 Turbellaria. According to the above remarks, the absence of intestine 

 in the Acanthocephali can scarcely be regarded as an important dis- 

 tinction. But the proboscidiform apparatus also, although of compli- 

 cated and peculiar structure, cannot form an objection to the existence 

 of a relationship with the Nematodes, since we are acquainted among 

 the Cestodes both with forms provided with and devoid of a proboscis 

 (e.g., Bothriocephalus). 



In conclusion, we may remember that the Acanthocephali mfani- 

 fest also in respect of their histology many resemblances to the con- 

 ditions which obtain among the Nematodes. Among other things, 

 both agree in the structure of the muscular fibres and the ganglia, in 

 the cuticular character of the connective tissue, in the often colossal 

 size of their cells, and in the complete absence of ciliated epithelium. 

 On consideration of these facts, it becomes evident that the Acantho- 

 cephali must be regarded as peculiarly modified Nematodes. The 

 relations of these two groups may be rightly compared to those which 

 obtain between the tape - worms and Trematodes ; that is to say, 

 the Acanthocephali may be regarded as forms of Nematodes which 

 have adapted themselves to the parasitic conditions of existence, 

 to a higher and more complete degree than the others. The 

 character of the young forms agrees with this conception, and we 

 are led to believe that they are more closely related to the original 

 conditions, because they are (according to my 

 observations) provided with the rudiments of an 

 intestine, 1 in which one can discern, notwith- 

 standing its incomplete differentiation, a pharynx 

 and an intestine. An oral aperture is wanting ; its 

 place is occupied by a grove in the form of a slit, 

 surrounded by a varying number of setae, em- 

 bedded in the retractile cephalic extremity (Fig. 

 72). On comparing this young form with the 

 common embryonic forms of the Nematodes, it 

 would seem as though the above asserted similarity 

 FIG. 72. Embryos wcre on ^ y a slight one ; but this opinion changes 



of Eclimvrhynchus an- ? P 5 



c/ustatus;A. the profile; when we consider the embryos of the genus Gor- 

 B. ventral view. ^^ ^ n w hi c h we mee t w fth relations (see specially 

 the illustrations published by Villot) which in fact differ only very 

 little from those of the embryos of Echinorhynclius. Gordius is a 



1 See Vol. II. 



