Introduction to Animal Morphology. 247 



parasites.* They can be arranged in two divi- 

 sions : — 



Division i. Brachiopoda {Diimeril)\ — marine 

 molluscs, usually small,J included in a bivalve shell 

 whose line of articulation is haemal, transverse, and 

 whose valves are ventral and dorsal, equilateral, but 

 dissimilar,! the ventral being usually the more convex, 

 overhanging the dorsal at the hinge, and often having 

 a longitudinal median furrow, while the dorsal has a 

 median ridge. The valves are opened|| and closed 

 by muscles, and have no elastic ligament, and the 

 beak of the ventral valve is often pierced by a hole^f 

 for the passage of a pedicle by which the shell is 

 attached to foreign bodies, or the surface of the ventral 

 valve may be itself adherent (Crania, Thecidium). 

 All the adult forms of living species are fixed.** 



The shell is generally dull-coloured,ff slow-growing, 

 laminated, and marked on the surface with concentric lines 

 of growth, Jl often with radial ridges, alternate on the opposite 

 valves, sometimes (in fossil forms) with hollow spines. The 

 shell has no nacreous layer, but consists of elongated, parallel, 



* Vulsella, Coralliophila, Magilus, Gastrochaena, are found in corals, 

 Stylifer in Starfishes, Eulima in Holothurians, Entoconcha mirabilis in 

 Sjnapta, &c. 



t Palliobranchiata (^Blainville), Brachionopoda {Brown), Spiro- 

 bzanchiata [Schmarda). 



X From o.oi" to 3" in length ; often as broad as long. 



\ The valves are scarcely distingmshable in Lingula. 



II But in the hinged forms can only open for a very small distance, in 

 Waldheimia from J" to -^", unless violence be used. 



IT The opening may be in the back, between it and the hinge, or be- 

 tween the two valves (Lingula). 



** When set free the dorsal valve, being heavier, is downward. 



tt Usually grey or brown. Waldheimia picta is spotted; Rhynchc- 

 nella is very dark ; Lingula green and bro\vn ; Crania white. 



XX Sometimes lamellose, as Orbicula, Crania. 



