552 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



forms. It is not unlikely that the Guinea worm (Filaria 

 medinensis), which infests the integument of Man in hot cli- 

 mates, may answer to the hermaphrodite stage of a similarly 

 dimorphous Nematoid, though its multiplication has hitherto 

 been supposed to take place agamogenetically. 



The many points of resemblance between the Nematoi- 

 dea, the Oligochceta, and the Polychceta, have been indicated 

 by Schneider. They differ, however, from these no less than 

 from the Turbellaria and Potifera, in possessing only longi- 

 tudinal parietal muscles. In this respect they agree with 

 Phamphogordius and Polygordius (united by Schneider into 

 the group of Gymnotoma), 1 which are segmented worms, 

 devoid of seta?, but possessing mesenteries, segmental organs, 

 and pseud-haemal vessels. Polygordius has a telotrochous 

 larva, and in its development, as in other respects, it is ex- 

 traordinarily like a polychaetous Annelid. 



Butschli, 2 on the other hand, dwells upon the connection 

 between the JSfernatoidea and the Gasterotricha {see Chap. 

 IV., p. 170) and Atricha {Echinoderes\ which he includes in 

 the group of N^ematorhyncha, on the one side, and the lower 

 Arthropods, such as the Tardigrada, on the other. 



The Physemaeia. — Since the completion of the third 

 chapter of this work, Haeckel 3 has published an account of 

 certain low Metazoa, constituting the two genera, Haliphy- 

 sema and Gastrophysema, which had previously been con- 

 founded, partly with the Sponges and partly with the Pro- 

 tozoa. 



These are minute marine bodies, having the form of cups 

 with longer or shorter stalks, by which they are attached. 

 The cavity of the cup into which the wide or narrow oral 

 opening leads is either simple (HaVphysema) or divided by 

 circular constrictions into two or more communicating cham- 

 bers ( Gastrophysema). The wall is composed of two layers, 

 an ectoderm and an endoderm — the latter being- formed by a 

 single layer of flagellate cells, like those of sponges ; and a 

 series of larger flagellate cells are disposed in a spiral, on the 

 inner face of the endoderm near the mouth. The ectoderm 

 is a syncytium, which attaches foreign bodies, such as sponge 



1 See supra, p. 165, note. 



2 u Untersuchuncren iiber freilebende Nematoden und die Gattung Chceto- 

 votus." (Zeitschrift fur wis*. Zoologie," 1876.) See also Ludwig, " Ueber 

 die Ordnunsr Gastrotrieha " (ibid.). 



- 3 " BioloirUche Studien," Heft 2, 1877. 



