68 CRISTATELLA MUCEDO. 



anatomy of an exserted Polyzoon is shown ; in Fig. 4 the same 

 retracted ; while Fig. 5 is the typical form of Timicata, with its 

 extraordinary respiratory apparatus, marked G. This consists of a 

 membranous sac, with transverse and longitudinal bars, crossing 

 each other and forming a series of quadrangular apertures, around 

 the interior of which vibratile cilia are placed. At V., the 

 imperfect, heart-like structure is found, and the respiratory organ 

 is connected with it by a system of vessels which bring the blood 

 to the sac for aeration. According to Allman, this respiratory sac 

 is homologous with the tentacular crown of the Polyzoa (A., Fig. 3). 

 He considers that the transverse, and not the longitudinal bars 

 are homologous to the tentacles of the latter. The Polyzoa are 

 destitute of blood, but its place is supplied by a fluid which fills 

 the whole of the perigastric space, and which extends into the 

 tentacula, having irregularly-shaped particles of matter floating 

 within it. This fluid assumes a constant rotatory motion, pro- 

 bably caused by the internal cilia which are supposed to clothe the 

 sac-walls. No apertures for the absorption of the watery fluid 

 have yet been discovered, and Allman states that he has kept 

 Cristatella for many hours in carminated water without detecting 

 any carmine particles within the perigastric space. He is never- 

 theless of opinion that the interior fluid is purely aqueous, and 

 that by some undetected orifices the water finds an entrance. 

 When there, it serves the triple purpose of a chyliferous, a sangui- 

 ferous, and a respiratory system. 



Having, thus briefly, endeavoured to show the important 

 differences in the two classes of Molhiscoida, I shall now confine 

 myself to the Polyzoa, or Bryozoa, which latter name originates 

 from the Greek " Bryon " — moss, and " Zoon," an animal ; the 

 habit of many of this class being to incrust foreign bodies like 

 moss. According to Professor Allman, the Polyzoa are divided 

 into two orders : ist, the ^^ Phylaciolcemata" or "throat-guarded," 

 so called from a curious valve of a protective nature, which 

 guards the mouth of the creature, and is situated in the crater of 

 the tentacular crown ; and 2nd, the " Gymnola;mata" or "naked- 

 throated," where no such appendage exists. The first order 

 includes all the Fresh-water species excepting Paludkclla, but 

 none of the marine except Pedicellifia. The 2nd order is made up 



