184 



IN VERTEBRA TE MORPHOLOQ Y. 



CHAPTEK IX. 



ORDER ECHINODERA; CLASS CHyETOGNATHA ; CLASS 

 ROTIFERA ; ORDER GASTROTRICHA ; DINOPHILUS. 



THIS chapter includes a description of a number of forms 

 whose affinities are at present rather doubtful and which 

 show similarities sometimes to the Neniathelmmths and 

 sometimes to the Annelida. Instead, however, of assigning 



them to one or the other of these types, 

 it has been thought advisable to consider 

 them in a separate chapter and each 

 group independently, indicating briefly 

 their most probable affinities. 



Order Echinodera. 



The order Echinodera includes a 

 number of small organisms all marine in 

 habitat, and all referable to a single 

 genus, Echinoderes (Fig. 91). The body 

 varies in length from somewhat less than 

 1 mm. to almost 0.1 mm. according to the 

 species, and tapers somewhat posteriorly, 

 terminating in one or two prolongations 

 or cerci, while anteriorly there is a pro- 

 FIG. 91. -- Echinoderes boscis armed with strong setse which 

 Dujardinii (after CLA- m ^ i nva p-i Da t e d within the anterior 



PAREDE from HATSCHKK). 



portion of the body, and serves as an or- 

 gan of locomotion as well as for the prehension of food. 

 The outer surface is covered by a layer of chitin which is 

 divided into distinct metameric rings, the number of which, 

 eleven, is constant for all known species, and which are pro- 

 vided in some species with definitely-arranged setae. No 

 cilia are present. Beneath the chitinous rings lies the ecto- 

 derm, which shows indications of metamerism also, being 



