PHANEROCEPHALA. 



475 



one at each end of the body, constituting a 

 preoral and perianal ring (Telotrocha, e.g., 

 Spio, Nephthys larva). In addition to these 

 two rings of cilia, incomplete rings jnay also 

 be present on the ventral surface (Gastro- 

 trocha), or both ventrally and dorsally 

 (Amphitrocha). In other cases one or more 

 rows of cilia surround the middle of the 

 body (Mesotrochd), while the terminal rings 

 (preoral and perianal) are absent, e.g., Chae- 

 topterus larvae (Fig. 379). Many larvae are 

 provided with long pro- 

 visional setae, which are 

 later replaced by permanent 

 structures, e.g. , the Mitra- 

 ria larva (Fig. 381), and 

 the larva of Nerine, etc. 

 In some cases the larvae 

 have true mesoderm seg- 

 ments, each of which is 

 provided with a ring of 

 cilia (Polytrocha, Fig. 383). 

 In spite of their great 

 diversity of form the Chae- 

 topod larvae can in their 

 later development also be 



reduced to the type of the larva of Loven (larva of Polygordius, 



Fig. 454). 



Branch A.* PHANEROCEPHALA. 

 Prestomium free and exposed, generally with eyes, tentacles, and 



palps. The body segments are more or less alike. 



Sub-order 1. NEREIDIFORMIA. 



With well-developed tentacles and palps ; the peristomium generally 

 possesses cirri. The parapodia are well-marked locomotor organs, 

 supported by acicula ; they carry dorsal and ventral cirri. The 

 setae are usually jointed ; uncini are never present. There is an 

 eversible buccal region and a muscular pharynx, which is usually 



* The classification here adopted is that of Dr. W. B. Benhara in his 

 admirable account of the Polychaeta in the Cambridge Natural History, 

 Worms, Rotifers, and Polyzoa, Macmilkn & Co., London, 1896. 



FIG. 382. Annelid larva 

 with provisional setae 

 (after Agassiz, from Bal- 

 four) . 



FIG. 383. Polytro- 

 chal larva of Oph- 

 ryotrocha puerilis 

 (after Clap, and 

 Metsch.). d intes- 

 tine; /ojaws. 



