SKELETONS, SHELTERS AND SPECIAL DEFENCES 653 



by a skeletal layer known as the ectocyst. This is flexible and horny in some 

 species, sometimes strengthened with sand grains, etc., and rigid and hard 

 when heavily impregnated with calcareous matter. The organic substratum 

 of the exoskeleton, opercula, etc., contains chitin (616). 



Encrusting or fern-like forms contain much organic matter (35-85%), 

 and have predominantly organic skeletons, e.g. Flustra. Others, largely 

 calcareous, such as Cellepora, contain about 5% organic matter (Table 

 15.1). Skeletal mineral matter is predominantly CaC0 3 . 



Tunicates. All tunicates are covered by a test of some sort, which is 

 usually composed in large part of tunicin (cellulose). In simple ascidians 

 the tunic has a leathery or cartilaginous consistency, whereas in compound 

 forms the test exists as a gelatinous or viscous matrix. Its main constituent 

 is cellulose. The cartilaginous test of Ascidia is about 90 % water, compared 

 with 65 % in the leathery test of Pyura. Analysis of Ciona test reveals 

 cellulose 60-3%, nitrogenous material 27% and inorganic matter 12-7% 

 (119). 



Test material is constantly sloughed off and lost at the surface, and new 

 test is secreted next to the epidermis. Secretion of tunicin is an epidermal 

 function, but mesenchyme cells are involved in growth and maintenance 

 of the tissue. Denuded areas of tunic are quickly replaced and regenerated 

 through the activity of epidermal cells (3). 



Of the pelagic tunicates (Thaliacea), Pyrosomas are colonies existing in 

 gelatinous tubes, doliolids and salps bear delicate barrel-shaped or 

 cylindrical tests. Older analyses for the tests of Pyrosoma show: water 

 content, 94-8%; cellulose, 1-2%; nitrogenous material, 3-2%; and ash, 

 0-7%. 



The tubes of the Pogonophora contain chitin. 



TUBES AND EXTERNAL CASES 



Tubes and cases are secreted and constructed by many kinds of animals. 

 They are usually characteristic of sedentary species, but we find some active 

 species that drag their cases along with them, and pelagic species that 

 inhabit special shelters. These structures are built of most diverse materials, 

 of organic and inorganic origin. In some species they are entirely secreted; 

 in others, built of environmental materials which are cemented together. 

 Tubes and cases afford shelter and protection ; they also form an integral 

 part of respiratory and food-collecting mechanisms in various species. 



Calcareous Cases and Tubes 



Calcareous tubes are characteristic of serpulid polychaetes. Other 

 instances of animals fabricating calcareous cases are the paper nautilus, 

 which makes a limy shell, and the shipworm, which deposits a calcareous 

 lining in its burrow. Calcareous shells and cases of foraminifers, molluscs, 

 etc., are described under exoskeletons in another section (p. 635). 



Serpulids are housed in tubes composed of a groundwork of so-called 



