652 



THE BIOLOGY OF MARINE ANIMALS 



Glottidia). In contrast are testicardinate and craniid brachiopods, the 

 valves of which contain less than 4% organic matter. Analyses of inorganic 

 constituents show that brachiopod shells fall into two groups ; calcareous 

 shells, and shells with high phosphate content (Table 15.1). In ecardinate 

 brachiopods, whose shells contain much organic matter, the predominant 

 mineral constituent is calcium phosphate; in craniids and testicardines, 

 calcium phosphate is replaced by calcium carbonate. The nature of the 

 organic matter in the shell is imperfectly known. Chitin has been identified 

 in the shell and pedicle of Lingula (ecardine) and the pedicle of Terebratalia 

 (testicardine). 



The brachiopod shell is secreted by the outer epithelium. It consists of 

 an external cuticle secreted by a chitinogenous band in the posterior region 



(C) 



Fig. 15.8. Structure of Brachiopod Valves 



(a), (c) Vertical section, (b) surface view; (a), (b) MageUania (testicardine); (c) Lingula 

 (ecardine). 1: periostracum ; 2: outer calcareous layer; 3: canal; 4: inner calcareous 

 layer; 5: radiating canaliculi; 6: chitinous layers; 7: calcareous layers. ((«), (b) from 

 King, 1870; (c) from Helmcke after Blochmann.) 



of the body, and underlying layers of mineral and organic matter. The shells 

 of different families differ somewhat in structure (Fig. 15.8). In lingulids 

 the shell consists of alternating layers of chitinous material and Ca 3 (P0 4 ) 2 . 

 In discinids the shell is composed of parallel chitinous lamellae 

 impregnated with Ca 3 (P0 4 ) 2 . Shells of Craniidae are composed of fine 

 organic lamellae interspersed with thick layers of CaC0 3 . Outside is a 

 thick cuticle. In the testicardinate families Terebratulidae and Rhyn- 

 chonellidae the shell has three layers : outer cuticle, thin mineral layer and 

 thick prismatic layer. The cuticle consists of organic ground substance 

 impregnated with CaC0 3 . 



Polyzoa. Polyzoans are usually colonial in habit, and the individuals are 

 enclosed in small cells which are aggregated to form colonies. In encrust- 

 ing forms like Membranipora and Flustra the cells are closely packed to- 

 gether in a single layer. Others form slender branching growths, e.g. Bugula, 

 or coral-like colonies, e.g. Heteropora. Externally the body wall is bounded 



