ENDOSKELETON. ANIMALS. 15 



cartilage, each forming an elongated spiral ; they are joined 

 to one another on the dorsal region of the worm by a narrow 

 bridge. Long slender rods arising from the anterior edge 

 of each scroll pass up the centre of the branchial filaments, 

 and from these rods smaller skeletal cores enter the indi- 

 vidual pinnules. The skeleton is composed of a variety of 

 cartilage, consisting of large vacuolated cells whose walls 

 are formed by a small amount of firm substance. 

 Viallanes, Ann. Sci. Nat., ser. 6, t. xx. 1885, p. 1. 



BRACHIOPODA. 



A. 49. The dorsal and ventral valves of a Brachiopod (Magel- 

 lania septigera). The shell consists of organic matter 

 hardened by calcium carbonate and a considerable propor- 

 tion of calcium phosphate. A complicated process (loop) 

 springs from the dorsal valve close to the hinge; it gives 

 support to the " arms," and functions as an endoskeleton. 



CRUSTACEA. 



A. 50. Endophragrnal system of a Lobster (Homarus vulgaris), 

 Infoldings (endopleurites) of the calcified cuticle project 

 inwards from the sides of the body in the intervals between 

 the segments. Each has a backward-directed process that 

 divides the area corresponding with the limb into an in- 

 ternal region lodging the protractor muscle, and an external 

 for the retractor. One of the endopleurites has been 

 painted light blue on its posterior face, the succeeding 

 endopleurite dark blue on its anterior. Between the 

 ventral plates (sterna) arise similar vertically directed bifid 

 processes (endosternites), painted red ; they articulate with 

 the endopleurites, and with one another in the mid line. 

 The two articular surfaces for the limb are painted black. 

 The sterna and their processes form a canal for the pro- 

 tection of the ventral nerve-cord and artery, and, with the 

 endopleurites, give attachment to muscles. 



A. 51. Ventral portion of the antepenultimate segment of the 

 cephalothorax of a Lobster (H. vulgaris). The endo- 

 pleurites and endosternites have each been divided 



