NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 39 



C, D), but the sacs themselves are eversible by muscles in their own 

 walls (C, i^, i8). A retractor {15, ip) arising within the para- 

 podium is inserted on the distal part of each chaetal sac, and a muscle 

 (ij, I'j) from the acicula, attached on the base of the sac, opposes 

 the muscles (j^, iS') that evert the sac itself. 



The parapodia are subject to numerous structural modifications in 

 the different groups of Polychaeta, and among the specialized types 

 the small leglike parapodia of the Myzostomidae (fig. 16 E) are of 

 particular interest because of their resemblance to the legs of Ony- 

 chophora. Each myzostomid appendage, as described by Stummer- 

 Traunfels (1903), contains a deep apical pouch (D, chS), from the 

 inner end of which a large hooked process (C/i) projects outward, 

 while from its distal wall a thick rod (Acic) extends inward and 

 gives attachment to protractor muscles (acmcls) and muscles inserted 

 on the base of the hook. It is evident that the hook is a single, greatly 

 enlarged chaeta, and the internal arm an acicula. The myzostomid 

 "leg," therefore, is only a modified parapodium adapted for clinging 

 to the crinoid hosts on which the Myzostomidae live, and has only a 

 superficial likeness to the appendages of Onychophora (fig. 31). 



THE NERVOUS SYSTEM 



The central nervous system of the Polychaeta, as shown in the 

 larval development, is produced from separate prostomial and somatic 

 rudiments, which secondarily become united (fig. 9) ; in the Oligo- 

 chaeta the two parts are said to be continuous from their inception. 

 The definitive brain, whether formed from discrete ganglionic centers, 

 as in Lopadorhynchus, or from a single generative zone of the pro- 

 stomial ectoderm (fig. 10 A, B, Br), is always a compact organ, 

 though it is generally bilobed (fig. 17 A) or differentiated into several 

 consecutive parts (fig. 18 C). The ventral nerve cords in the more 

 primitive condition found in most of the archiannelids and in various 

 polychaete and oligochaete families are entirely separate, except for 

 their connection by commissures (fig. 19 A, B), and in such cases 

 the nerve tissue usually preserves a close contact with the ectoderm 

 from which it is derived (C). More commonly, however, the paired 

 ganglia of the cords are united in single median ganglia (fig. 17 C, D), 

 giving the cords themselves a median position ; but even in such cases 

 the ganglia of one or more pairs carried by the divergent anterior 

 ends of the cords may remain widely separated. The first pair of 

 united ganglia on the cords constitutes the so-called "suboesophageal 

 ganglion," but it is evident that this ganglion does not belong always 



