CLASSIFICATION. 19 



trial types. Alimentary canal bent upon itself, the rectum 

 very commonly opening into the mantle cavity above the 

 cephalic portion of body. (Huxley.) Nervous system, same 

 plan as Lamellibranchiata tendency to aggregation of gan- 

 glia toward cephalic extremity. Foot constantly present. 



Pteropoda. Shell univalve, if any. Alimentary canal and 

 nervous ganglia resembling in arrangement those of Gastero- 

 poda. Cephalic extremity rudimentary. 



Cephalopoda. Shell uuivalved, either elaborate, rudiment- 

 ary or absent. Respiratory apparatus by gills, either in one 

 or two pairs. Alimentary canal and nervous ganglia as the 

 last two groups. The margins of foot are produced into 

 more or less numerous tentacular appendages, often pro- 

 vided with singularly constructed suckers or acetabula, the 

 mouth being situated at a central point 



RING (Articulata). 



The embryo of an articulate is evolved from the ventral 

 aspect of the ovum, and presents serrated borders. In adult 

 condition articulates present a series of rings joined together 

 by intervening tissues. In the lower groups, as Rotifera, 

 Entozoa, and Annelida, this arrangement is less evident than 

 in the higher; hence these animals are by some separated 

 from the Articulata and termed the Annulosa. A single 

 ring of an articulate animal thus characterized contains a 

 portion of the main blood-vessel, nervous system and ali- 

 mentary canal, arranged in the following order: blood vessel 

 under dorsal surface, nervous system on the ventral, and the 

 alimentary canal between. At the cephalic extremity of the 

 animal, this arrangement is modified by the nerves at that 

 point forming a collar or ring (cesopbageal ring) around the 

 gullet, which then terminates toward dorsal aspect of seg- 

 ments. The organs, as a rule, are arranged upon either side 

 of a longitudinal axis. Divided into Rotifera, Entozoa, An- 

 nelida, Crustacea, Myriapoda, Arachuida, and Insecta. 



Rotifera. Aquatic, limbless articulates. Cephalic ex- 

 tremity furnished with a retractile (often lobed) disk, upon 

 which are usually placed cilia. Water vascular system 

 present. 



