28 INVERTEBRATE ZOOLOGY. 



Order 3. Gnathobclellidae Pharynx not extensile, with three longi- 

 tudinal ridges which are often toothed. 

 ■iic HivKdo. 



SUPPLEMENTARY CLASSES. 



Class Chaetifera, formerly taken with Sipnnculoidea to form class Gephy- 

 rea. Here belong a very few marine worms, Echiiirvs, Bonellia, etc., which 

 show affinities to the Chaetopoda. They are segmented only as larvae, but 

 possess paired nephridia and a sj^stem of blood vessels similar to that of 

 Annelids. 



Class Sipunculoidea includes a very few forms of marine worms, l)earing 

 some slight affinity to Annelids. They were formerly united with Chaetifera 

 to form the class Gephyrea. They are cylindrical, iinsegmented forms with- 

 out bristles, and possess a retractile proboscis. -^ SijninciiJtis, Pliascolosovia. 



Class Chaetognatha. This includes one form, Sagitta, a small, transpar- 

 ent, unsegmented worm found on the surface of the ocean. The body is flat 

 and possesses lateral fire-like extensions. The mouth is armed with jaws beset 

 with sharp hooks. Hermaphroditic. It develops lateral coelom sacks, which 

 appear to have the value of a inetacoelom. It is placed liy some with the 

 Nematodes. 



Sub-cladus II. Arthropoda. 



Class I. Crustacea Aquatic Arthropoda. a few being second- 

 arily adapted to a terrestrial life. Res- 

 piration either through the general sur- 

 face of the integument or by localized 

 thin portions of the same, in the form of 

 evaginated plates or structures, almost 

 always placed in some relation to the 

 appendages — and known as " gills." Ajv 

 pendages typically composed of two 

 branches, which may be modified beyond 

 recognition. Development from a Nau- 

 plias larva, which is suppressed in the 

 higher forms. Female generally pro- 

 vided with a brood-sack for the care of 

 the young. 



