156 INVERTEBRATE ZOOLOGY 



while the latter have jaws. Species of the genus Piscicola which 

 live on fishes, Glossiphonia, and Placobdella are characteristic 

 of the first order, while the medicinal leech {Hirudo medicinalis) 

 and Macrobdella, which so commonly attacks bathers, are 

 examples of the Gnathobdellida, 



Class Archiannelida 



As the name signifies, the Archiannelida seem to represent 

 a primitive type of annelid organization which may have con- 

 siderable significance in solving the problem of the origin of the 

 higher annelids. The body, which shows only slight indica- 

 tion of segmentation externally, has a coelom completely divided 

 into somites. Both parapodia and setae (Fig. 76 E) are lacking. 

 The nervous system is distinctly more simple than that character- 

 istic of other annelids, for it remains in direct contact with the 

 epidermis and shows no centralization to form ganglia. In the 

 genus Protodrilus, there are two ventral nerve cords connected 

 by transverse commissures, but in Polygordius there is a single 

 cord. Representatives of both of these genera are exclusively 

 marine. The prostomium bears a pair of tentacles and in addi- 

 tion to these a pair of ciliated grooves are the only structures 

 which seem to have a sensory function. 



In the development of Polygordius, a typical trochophore 

 (Fig. 76) occurs the formation and metamorphosis of which have 

 been worked out in great detail. 



Appendix to the Archiannelida 



Dinophilus and some other simple wormlike forms are of 

 questionable systematic position. Some zoologists maintain 

 that they show possible relationships with the Archiannelida, 

 while others consider them as more closely related to the Trochel- 

 minthes. Members of the genus Dinophilus are minute marine 

 worms which live among seaweeds. The body consists of a head, 

 five or six trunk segments, and a tail segment. The adult worm 

 rather closely resembles the larva of marine polychaetes. 



Class Gephyrea 



The Gephyrea are marine worms which differ from the remain- 

 ing annelids in the lack of segmentation, parapodia, and setae. 

 Development involves a modified trochophore larva. The 



