Characteristics of Gephijrea. cxxxi 



provisional organs, in the shape of terminal or biterminal (telo- 

 trochal) or mesially-placed (mesotroehal) zones of cilia ; and some- 

 times in that of long- setae. In every ease the cephalic and caudal 

 segments alone are present when the Polychaetous embryo leaves 

 the egg ; and their full complement of segments is attained to 

 in the way of intercalation anteriorly to the terminal segment. A 

 few Polychaeta belonging to the genera Eunice, Nereis, Syllis, 

 have been observed to be viviparous. The Discoj)hora differ from 

 the other Annulata, in never multiplying asexually ; and in pos- 

 sessing but little power of repairing injuries, and none of repro- 

 ducing lost organs. In these points the DiscopJwra differ very 

 widely from the Platyelminthes, with which they have sometimes 

 been classed (see p. 138, infra), and resemble the Nematelminthes, 

 a Class to which also they are approximated by the possession of 

 a terminal sucker, though by few other properties beyond those 

 conjmon to all members of the Sub-kingdom Vermes. 



Class, Gephyrea. 



Marine worms, which have been supposed, as their name denotes, 

 to furnish an example of a transitional form connecting two distinct 

 types, and which not only in habits of life and in external appear- 

 ance, but also in a few points of internal structure, do appear to be 

 allied to the apodal Holothurioidea, though the totality of their 

 organism causes us to class them amongst the Vermes, and to 

 consider tliem as most nearly related to the Annulata proper. They 

 are usually cylindriform in outline, their integumentary system is 

 indurated by chitinous, but never by calcareous deposit; in two 

 families [Sternaspidea and Ec/iiuridea), it carries locomotor setae; 

 and in one species at least {Anoplosomaiuiu antlllense) of another 

 family {Priapulaceae), it developes trichocysts. It ordinarily pos- 

 sesses numerous follicular glands. The Gephyrea never have their 

 bodies definitely segmented, but the integument is not rarely so 

 corrugated as to form more or less distinct zonular folds; and in one 

 case {Phascolosoma Cimanense°), rudimentary dissepiments have 

 been observed in the large perivisceral cavity. The mouth is 

 surrounded in the SipuiLCididae with a coronet of ciliated tentacles, 



•> Keferstein, Zeitschrift fiir Wiss, Zoologie, xvii., 1867, p. 53. 



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