184 



INVERTEBBATM MORPHOLOGY. 



CHAPTER IX. 



OEDER ECHINODEEA; CLASS CH^TOGNATHA ; CLASS 

 EOTIFERA ; ORDER GASTROTRICHA ; DINOPHILUS. 



This chapter includes a description of a number of forms 

 wliose affinities are at present rather doubtful and which 

 show similarities sometimes to the Nemathelminths 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 anterioi-ly there is a pro- 

 ■Wia. ^\. — Echinoderes hoscis, armed with strong set^ which 

 fARtoltom nl^sln^'. ™ay be invaginated within the anterior 

 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 setse. No 

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

 derm, which shows indications of metamerism also, being 



