CHAP. VIII.] EPITHELIAL TISSUES, &C. 299 



SECT. 2. TISSUES WHICH YIELD CHITIN, SPONGIN, TUNICIN, 

 AND HYALIN. 



The tissues of many groups of invertebrate animals contain 

 certain of the proximate principles which have been enumerated and 

 described as obtained from the tissues of man and the higher verte- 

 brates. Thus mucin is present in organisms low in the scale; as 

 we ascend, we find chondrin-yielding tissues, and in the Cephalopoda 

 tissues which yield gelatin 1 when boiled. 



In certain invertebrates we find, however, that the tissues contain 

 substances which do not occur in vertebrates. Amongst these are 

 the substances now to be considered, viz. Chitin, Conchiolin, Spongin, 

 Tunicin, and Hyalin. 



Chitin. 



Distribu- Chitin usually occurs throughout Invertebrates in the 



intne Animal f rm f an investment to the outermost cellular layer 

 Kingdom. or ectoderm. The exceptions real and apparent to this 



statement are noted in the following table of its distribution, which 

 however must be accepted as only approximately accurate, in the 

 absence of full chemical investigation, in any case except that of the 

 Arthropoda 2 . 



Protozoa. Membrane of all "loricate" forms, cuticle of Infusoria, 

 &c. Oesophageal lining of toothed Ciliata (Nassula, Prorodon). 

 Central capsule of Radio! aria. Cyst wall of all encysted forms. 



Coelenterata. Membrane of fertilized ovum. Mesodermal (?) 

 skeleton of Hydro-medusae (Velella). 



Vermes. Membrane of ovum. Cuticle in all cases, including 

 the ectocyst of Polyzoa, and cuticular appendages, such as the 

 setae of Annelida. Oesophageal armature of Rotifera and some 

 Annelida. Mesodermal branchial skeleton of Balanoglossus. 



Echinodermata. The presence of chitin is not indicated in this 

 group. 



Mollusca. Membrane of ovum. Setae of larval Brachiopoda. 

 Byssus, shell-ligament and shell (in many cases, if not universally, the 

 organic base of the shell is composed not of chitin but conchiolin, 

 q. v.). Rings and hooks of suckers of Cephalopoda. Upper lip and 

 jaws of Cephalopoda and Gasteropoda. Radula of odontophore. 

 Mesodermal branchial skeleton of Lamellibranchiata. 



1 Hoppe-Seyler, "Ueber Unterschiede im chemischen Ban und der Verdaunng 

 hoherer und niederer Thiere." Pfliiger's Archiv, Vol. xiv. p. 395400. "Ueber das 

 Vorkommen von leimgebendem Gewebe bei Avertebraten." Med. Chem. Untersuchung . 

 p. 580. 



2 It would seem that in many cases a chitinous composition has been ascribed to a 

 structure solely on the ground of its insolubility in caustic alkalies and dilute acids, or 

 even in only one of these two classes of reagents. 



