Genera of the IsTorth American Palaeozoic Bryozoa. 441 



an elaborate prehensile appendage, charged with a special ser- 

 vice in the interests of the colony. 



"In the articulated avicularium, the 'bird's head' is supported 

 on a short peduncle with a basal joint, on which it sways to and 

 fro. The head is composed of two portions, a lower which is 

 more or less rounded above, and forms the chambers for the 

 muscles (= the cavity of the zooecium), and an upper and anterior, 

 which consists of a movable mandible and a curved beak opposed 

 to it. This anterior portion is composed of horny material, 

 whilst the chamber itself is calcareous. The walls of the pro- 

 jecting upper jaw, which terminates in the hooked beak, inclose 

 an aperture, over which stretches a delicate membrane, pierced 

 by a small circular orifice. This aperture represents the mouth 

 of the zooecium, the mandible taking the place of the operculum. 

 Within the chamber occurs a small circular body, composed of 

 distinct cells, Avhich is connQc\,Q&{mBuguUJlabellata)\^\\h^o\x^- 

 shaped organ, opening out through the membrane of the aper- 

 ture. From the bottom of the cup rise a number of setae, which 

 project beyond the opening and constitute the tactile organ be- 

 fore referred to. The cellular body, in connection with the 

 setiferous cup has been regarded as a nervous ganglion (B sk, 

 Smitt); the two together constitute, according to Nitsche, the 

 homologue of the polypide, which is here reduced, in conformity 

 with the altered significance of the whole structure, into a mere 

 organ of touch. There can be but little doubt, I think, but that 

 the latter is the true view ; at the same time it must be regarded 

 as probable that the rudimentary polypide is furnished with 

 .its nerve center, by which the powerful muscular apparatus 

 and the sensitive organ may be supplied. Whether the cellular 

 body constitutes the ganglion, we are not at present in a posi- 

 tion to decide. The articulated avicularia are always attached 

 to the wall of the cell, and usually at a short distance from the 

 orifice; they are confined apparently to a small number of 

 genera. Some further evidence of the morphological nature of 

 these curious appendages may be briefly noticed. (I.) In some 

 cases I have met with oyicells developed over the upper extrem- 

 ity of the aviculariafii beak and mandible, clearly indicating their 

 morphological relation to the orifice of the zooecium. On more 

 than one occasion this lusus has occurred to me in Sehizotheca. 

 56 



