636 THE BIOLOGY OF MARINE ANIMALS 



other coverings. The plant-like flagellates are provided with a polysac- 

 charide membrane, which in dinoflagellates takes the form of stout plates 

 of cellulose. Other species are enclosed in shells bearing pores or apertures 

 through which the animal emerges or sends forth strands of cytoplasm. 

 Secreted tests of organic matter are found in foraminifers (Gromia, 

 Al/ogromia), folliculinid and tintinnid ciliates. Some of these tests are said 

 to have a nitrogenous basis and may be proteinaceous. Those of fora- 

 minifers are covered externally by a layer of cytoplasm. Foreign bodies are 

 frequently incorporated in the shell, e.g. tests of foraminifers, loricae of 

 tintinnids. The tests of the majority of foraminifers are calcareous, and 

 take the form of loose-fitting shells with wide mouths, or chambered shells 

 perforated by numerous pores. In some groups the skeleton is arranged in 

 the form of an external lattice, siliceous in Silicoflagellata and calcareous 

 in Coccolithophoridae. 



Siliceous foraminiferan tests are constructed largely of sand grains on 

 an organic base and contain 76-95 % silica. Calcareous foraminifers, such 

 as Orbitolites and Polytrema, have tests composed largely of CaC0 3 

 (86-89 %). Variable amounts of MgC0 3 are also present (2-16 %). The con- 

 tent of the latter is appreciably greater in warm-water species. Spectro- 

 scopic analysis also demonstrates the accumulation of surprisingly large 

 amounts of strontium and silica in calcareous tests. In the Xenophyophora, 

 deep-sea forms sometimes grouped with Foraminifera, the skeleton con- 

 sists of grains of barium sulphate (21, 23, 28, 114, 132). 



Coelenterates. An external secreted skeleton is a characteristic feature of 

 many sedentary coelenterates, notably the corals. 



The soft parts of hydroids are covered by a chitinous perisarc which is 

 secreted by the epidermis, probably by specialized glandular cells. This is 

 occasionally strengthened by deposition of calcareous matter, e.g. Hydrac- 

 tinia. The pneumatophore or air sac of siphonophores has a chitinous 

 lining. Chitin is said to be absent from the Anthozoa (2, 65). 



Corals. Millepore corals (Hydrozoa) possess a calcareous exoskeleton 

 which takes the form of leaf-like or branching growths. Tubes from the 

 polyps ramify through the skeleton, which corresponds to the perisarc of 

 hydroid colonies. Analyses of Millepora and of Distichopora, another 

 coralline hydroid, show that the skeleton consists largely of aragonite. 

 Organic matter forms about 3 % of the skeleton (dry weight) ; mineral 

 matter is predominantly CaC0 3 (Table 15.1). 



Alcyonaria are colonial in habit and are supported by a skeleton secreted 

 by mesogloeal cells. The skeleton is calcareous or horny in composition 

 and differs greatly in construction in different groups. Organic skeletons 

 consist of strands and lamellae of horny material. Calcareous skeletons 

 may consist of spicules embedded in a horny network, e.g. gorgonians; of 

 separate calcareous spicules, e.g. soft corals ( Alcyonacea) ; or of spicules 

 fused by calcareous cement, e.g. organ-pipe coral Tubipora. 



The skeleton of Alcyonium, a typical genus, consists of mesogloeal 

 spicules. These contain an organic axis, secreted by scleroblasts, on which 



