ANNELIDA 279 



segments are present ; new ones are, however, continually 

 being interpolated behind. Since, moreover, the parapodia 

 acquire more and more their permanent shape, and the 

 larval organs, on the other hand, degenerate, the larva 

 approaches the shape of the adult animal. 



The segmental appendages do not arise in a uniform manner in the 

 different divisions of the Polychaeta. In the Errantia the dorsal and 

 ventral parapodia arise from a common fundament, which afterwards 

 separates into the dorsal and ventral parts. This has been observed, for 

 example, in Nereis. Such a separation, however, does not take place in 

 the Sedentaria, but their dorsal hook-bearing segmental appendages arise 

 independently of the ventral parapodia (thus in Terehelhi). Accordingly 

 it is maintained that only the dorsal appendages of the Sedentaria cor- 

 respond to the common parapodial fundament in the Errantia, whereas 

 the ventral appendages are to be considered as new formations of a 

 secondary nature (Salensky). 



The cirri and tentacles arise as elevations and evaginations of the 

 ectoderm, into which continuations of the somatic mesoderm may also 

 extend. Of these the unpaired median tentacle, as it occurs, for example, 

 in Terebella, Pileolaria, and Psygmobranclms, usually extending forward 

 beyond the head, is of an especially peculiar nature. It attains at first 

 a large size, and is provided with a considerable cavity (Terebella), but 

 may soon become reduced in size again {Psygmobranclms). When it is 

 present, there are found near it, and on either side of the head, the 

 lateral tentacles, the number and form of which are very variable in the 

 different Annelida. The tentacles may attain a pecuhar development by 

 putting forth bud-like evaginations, which enlarge and become the gills. 

 In Psygmobra7ichns the larva, by means of the trifid gills, acquires quite 

 a peculiar shape (Fig. 128 B). ^ The median tentacle, which was present 

 somewhat earlier and extended forward beyond the head, has already in 

 this stage degenerated. As sometimes the tentacles, so also may the 

 eyes, degenerate in the Sedentaria, since these sessile forms can scarcely 

 have further need of them. In Psygmobranclms a peculiar organ is 

 seen lying behind the gills (Fig. 128 B, kr), which is also developed in 

 other Annelida (Pileolaria, Spirurbis). This is an annular fold of the 



1 The larva which is here figured has developed from a so-called 

 monotrochal larva (Fig. 128 A), which exhibits the form of the Trocho- 

 phore, provided with a preoral and post-oral ciliated band, while the 

 preanal band is wanting. The post-oral ciliated band is continued 

 into the ventral (so-called adoral) ciliated groove. The Troehophore 

 already possesses two eye-spots, but still lacks the intestinal canal, which 

 is represented by only an entodermal mass of large cells. The mouth 

 is already indicated. 



