56 PHYSIOLOGICAL SEKIES. 



water ; if the Membranipora were rigid it would be the 

 subject of frequent fracture ; this is prevented by each 

 calcareous lateral wall of the zooecium having two places 

 where it is membranous and flexible. The dorsal and 

 ventral walls are entirely membranous. PI. XI. fig. 2. 

 Nitsche, Zeitschr. wiss. Zool., Bd. xxi. 1871, p. 418. 



B. 45. Second antenna of a Lobster (Homarus vulgaris), with 

 drawing of its structure. The proximal segments can 

 alone be moved by muscles. The long and tapering distal 

 part (flagellum) is composed of calcified rings connected 

 by flexible noncalcified cuticle. This structure, that greatly 

 diminishes the risk of fracture, is common to all filamentous 

 antennse. The filamentous portion of the antenna that is 

 naturally straight has been coiled to show its flexibility. 



Joints that allow of voluntary motion. 



INYERTEBRATA. 



B. 46. Spines and plates from the test of a Sea-urchin (Hetero- 

 r.entrotus mammillatus, fam. Echinometradse), some in section, 

 with diagrams showing relation of soft parts. The spine 

 articulates by a ball-and-socket joint, the surface of the 

 socket being about half that of the mamelon. 



B. 47. A similar preparation and diagram of Phyllacantlms im- 

 perialis, fam. Oidaridse. A pit in the centre of the socket 

 and mamelon gives attachment to a round ligament. This 

 is also present in the Diadernatidse and Echinoidea Irregu- 

 laria. The middle spine in the preparation is from P. 

 baculosa, the dried soft parts are still attached, the round 

 ligament has been painted blue. 



Prouho, Arch. Zool. Exp., t. v. 1887, p. 252. 



B. 48. Separated dorsal and ventral valves of a Brachiopod 

 (Magellania lenticularis), with the hinge-region of another 

 specimen. Two processes of the ventral valve (one painted 

 blue) are locked in depressions (one painted red) in the 

 dorsal valve. The valves can only be completely separated 

 by fracture. The articulation admits of but slight motion. 



