RELATIONS OF THE ACANTHOCEPHALI. Ill 



as it was confined almost exclusively to the external formation of the 

 body. 1 The internal organization of the Sipunculidre shows scarcely 

 any close relation to the Acanthocephali, unless the presence of an 

 unsegmented dermal muscular tube be regarded in this sense. Also 

 the fact that the gulf between the two groups is not bridged over 

 by any intermediate forms, further lessens the probability of a re- 

 lationship between them. We know, however, thanks to recent 

 researches of a parasitic animal closely related to the Sipunculidae, 

 namely, the male of Bonellia, which (p. 10) lives as a parasite in the 

 sexual passages of the female ; but nothing in the animal betrays 

 approximation to the Acanthocephali. The structure reminds one 

 rather of the condition in the Planarians, or the ciliated embryonic 

 condition of other worms. Also the similarity to the peculiar genus 

 EcJiinoderes 2 is limited to certain external characters (the presence 

 of hooks upon a conical head), and does not justify the opinion of a 

 genetic connection. 



But though it must be confessed that no group of animals can be 

 adduced to which the Acanthocephali could be directly traced, this 

 fact does not, of course, in any way involve the conclusion that they 

 have no relation to any other forms. This only may be learned from 

 it, that these relationships, instead of being manifest as in other cases, 

 are of a more hidden nature; in other words, that the Acantho- 

 cephali are related to forms of animals which have succumbed to a 

 deep-seated modification before the typical structure of the parasites 

 in question was developed. The dropping out of the intermediate 

 members, of course, causes the position of these worms to appear very 

 isolated. If, from this point of view, we search for forms which might 

 be considered as the starting-point of the Acanthocephali, then our 

 attention will soon be drawn to the Nematodes, which like them are 

 parasitic. I will base nothing on the fact that there are thread- worms 

 which, being provided with a proboscidiform and armed cephalic ex- 

 tremity, have occasionally been considered as Echinorhynchi. An 

 erroneous interpretation cannot have the force of proof. But this would 

 have been almost impossible, had not so many other similarities ob- 

 tained between the two forms. In fact, both possess an elongated 

 cylindrical body, the walls of which are formed of a strongly developed 

 dermal muscular tube, surrounded by a firm integument. This tube 

 is traversed by longitudinal vessels, and encloses a distinct body- 



1 Schneider also attempts to support the relationship with the Sipunculidse by means 

 of the structure of the muscular apparatus, which in its arrangement differs from the 

 conditions found in the Nematodes, and agrees more with those of the Sipunculidse 

 (Mutter's Archivf. Anat. u. Physiol, p. 592, 1864). 



2 See especially Greeff, Archivf. Naturgcsch., Jahg. xxxv., Bd. i., p. 72, 1869 ; and 

 Pagenstecher, Zeitschr. /. iviss. Zool, Bd. xxv., Suppl., p. 117, 1875. 



