20 



RESPIRATION AND CIRCULATION. 



The tentacula are tubular, closed at their free extremity, and opening by the opposite 

 through the lophophore into the perigastric space ; in all the Polyzoa they are armed upon 

 their opposed sides with vibratile cilia, arranged in a single series, and vibrating towards the 

 remote extremity of the tentacle upon one side, and towards the base on the other. Two very 

 distinct layers (PI. IX, fig. 5) enter into the structure of the tentacula. The external layer 

 consists of rounded cells filled with a colourless fluid, and often presenting a bright nucleus. 

 Some of those cells which lie upon the back of the tentacle become in certain genera enlarged, 

 giving a vesicular appearance to the organ ; this is particularly evident in CristateUa. The 

 internal layer is a delicate transparent membrane, in which I could detect no trace of 



approximated at their extremities, and thus, instead of constituting an open crescent, they form a 

 ring enclosing a space which embraces within it the termination of the intestine. 



Fig. 3. 



Fig 



Plan op Pedicellina. 

 Fig. 3. Longitudinal section. Fig 4. Front view of lophophore and contiguous parts. 



Ectocyst. b. Endocyst. c. Invaginated portion of eudocyst adherent to the bases of the tentacula and entering 

 into the formation of the cup. d. Muscular fibres surrounding the margin of the cup. e. Mouth. 

 /. (Esophagus, g. Stomach, h. Rectum, i. Anus. k. Epistome. I. Ganglion. ;«. Generative organ. 

 n. Retractor muscle, o. Lophophore. 



The tentacula are connected to one another at their base by a membrane which adheres to their 

 back, extending forwards for about one third of their length, and constituting a cup. This membrane 

 passes uninterruptedly from the extremity of one arm of the lophophore to that of the other, thus binding 

 together the two tentacula wliich spring from the points of the arms, and completing the tentacular crater 

 so as entirely to disguise the hippocrepian character, and give to the crown of tentacula the form presented 

 by the infundibulate genera. The result of this arrangement is, that the anus presents the anomalous 

 condition of opening within the tentacular crater. It will be easily seen, however, that the position 

 of the anus though within the circle of tentacula is still properly external to the lophophore, and 

 thus really occupies its normal position in the concavity of the crescent. 



The membranous cup which surrounds the base of the tentacular crown is not homologous with 

 the calyx of the ordinary hippocrepian Polyzoa, but would seem rather to represent a permanently 

 invaginated portion of the endocyst, with which, however, a true calyx equivalent to that of the fresh- 



