214 



INVERTEBEATE ANIMALS. 



in communication with one another by a small tube or " siphuncle." 

 The shell thvis constituted is buried in the mantle at the hinder 

 end of the body (fig, 154). Lastly, in the curious extinct group of 

 the Behnniiites, which also belong here, the elongated body was 

 supported by a long dart-like internal skeleton, the chief portion 

 of which consisted of a cylindrical pointed fibrous body, called the 

 " guard." In front, the guard was hollowed out into a conical 

 cavity, in which was lodged a singular series of chambers, imited 

 by a tube or " siphuncle," and forming collectively what is called 



Fig. 15-. — a Internal skeleton of .sv^xVt ornaia ; h Pen of llistioteutJiis BoneUiana; 

 c Shell (" xjhragmacone") of Spirula /ragilis; d Animal of Spirula Peronii. 



the " phragmacone. " In fiont of that, again, the skeleton was pro- 

 longed forward into a, more or less extensively developed horny 

 expansion or "pen." 



The second order of the i'l'plialojjoiht — that of the Tetrabran- 

 chiata — comprises forms chaiacterised by being creeping animals, 

 protected by an exU-nial 'niniiii-flutrnhered shell, the partitions be- 

 tween the chambers being jjerforated for the ]jassage of a mem- 

 branous or calcareous tube, termed the "siphuncle." The arms are 

 more than ten in number, and are devoid of suckers ; the gills are 



