Characteristics of Nematelminthes. cxxxiii 



nerve-cords of the Echinodermata. There are no org-ans of special 

 sense in this Class^ with the exceptions of the tentacles of the Si- 

 puncididae ; and of the eye-specks retained by some members of 

 that family from their free larval into their adult life, which they 

 spend immersed in mud or sand. 



The Gephyrea are not hermaphrodite ; the organs formerly con- 

 sidered to be testes having been shown to be ducts for the gene- 

 rative products, and homologous with the segmental organs so 

 characteristic of the Annulata. The embryos go through more 

 or less complicated metamorphoses ; the provisional organs of the 

 larval form known as Adinotrocha, attaining proportions and rela- 

 tions to the future Sipunculus, which are similar to those of the 

 most characteristically developed Echinodermata ; whilst in other 

 cases the larval may differ from the adult form only by its possession 

 of locomotor ciliated zones, and of a relatively more prominent 

 nervous system. 



For the anatomy and classification of the Gephyrea, see Lacaze 

 DuthierSj Ann. Sci. Nat., Ser. iii., tom. x., 1858; Claparede, 

 Anatomic und Entwickelungsgeschichte Wirbelloser Thiere, 

 1863, pp. 61, 83; Keferstein und Ehlers, Zoologische Beitrage, 

 1861 ; Ehlers, Zeitschrift fiir Wiss. Zoologie, xi. ; Keferstein, 

 ibid, xii., p. '^^, 1 862 ; Semper, ibid, xiv., p. 420 ; Keferstein, 

 ibid. XV. and xvii., 1867. 



Class, ITematelminthes. 



Cylindriform non-segmented Vermes, in which the external layers 

 of the integument ordinarily attain considerable rigidity by chiti- 

 nization, and occasionally develope hooks and bristles, but in which 

 true locomotor more deeply implanted setae, and cilia are absent. 

 Three orders, the Ckaetognatha, Nonatoidea, and Acanthocephali, 

 are contained in this class, of which the first consists of a very 

 few species which are always free ; the second comprises a very large 

 number of species which may be either free or parasitic ; and the 

 third is exclusively entoparasitic, and devoid of any digestive 

 tract. 



Lateral and caudal fins, and suckers, as well as setiform spicula, 

 are often present in this Class ; and in the two orders, Chaeiognatha 



