Ixxxiv INTRODUCTION. 



The Polypide of the Entoprocta. 



In the group of tlie Entoprocta the polypide differs 

 so widely and essentially in structure from that of the 

 Ectoprocta that it is necessary to treat it separately. 

 Its chief peculiarities, however, are described elsewhere 

 (pages 563, 564) ; and I need only add that the import- 

 ance of the characters on which the new division is 

 founded has been generally recognized*. For an account 

 of the aberrant group of the Pterobranchia the reader is 

 referred to the systematic portion of this work (pp. 577- 

 580) . 



Organs of Sense. 



Special organs of sense are of rare occurrence amongst 

 the Polyzoa; and such as exist are of the simplest kind. 

 With a single exception they are all tactile in function, 

 and very similar in structure. 



The most elaborate occur in the genus Loxosoma, and 

 consist of papillae, placed one on each side of the body of 

 the polypide, bearing on the summit a number of rigid 

 setse. The papillae are contractile, and can be almost 

 wholly withdrawn t; their cavity, according to Salensky^ 

 is occupied by a nerve-knot, which is connected by 

 nervous filaments with the central ganglion. The setse 



* An extensive literature has sprung up devoted to the Entojirocta. See 

 Nitsche, " Ueber Pcdicellina eckinafa," Zeitsch. wissensch. Zool. xx. (1870), 

 Heft 1 ; "Ueb. den Ban, &c. von Loxosoma Keferstemii," ibid. xxv. p. 451 : 

 Schmidt, O., " Die Gattung Loxosoma," Arch. f. mikr. Anatomie, xii. : 0. 

 Vogt, "Sur le Loxosome des Pliascolosomes," Arch, de Zool. experimen- 

 tale, 1877 (transl. in Quart. Jonrn. Micr. Sc. (u. s.) vol. xvii. p. 353) : Sa- 

 lensky, " Etudes sur les Bryoz. Entoproctes," Ann. Sc. Nat. 6^ ser. Zool. v. 

 (1877), article no. 3: Hatschck, " Ueb. Fedicellina,'" Zeitsch. xxix. p. 502. 



t Vogt describes the papilla; as containing " elongated, conical, con- 

 verging cells, apparently in direct communication with tlie bristles." He 

 failed to detect tlie nerves. 



