388 



TEGUMENTARY ORGANS 



animals in the following order: — i. Hydroid and Actinoid Polypes 

 and Beroidae. 2. Annulosa, including the worms and Echinoderms. 

 3. Mollusca, including the Ascidians and Polyzoa. 4. Vertebrata. 



I. Hydroid and Actinoid polypes. — In these animals the integument 

 consists either of a simple cellular and vacuolated ecderon, or the 

 outer layer of this is developed into a structureless coat, which may 

 become thickened by repeated additions, and thus attain considerable 

 dimensions. In the common Campanularia, for instance, the outer 

 wall of the bud from which a polype is to arise consists, at first, of a 

 mass of indifferent tissue. As development proceeds, the outer 

 portion of the mass is converted into a structureless membrane, which 

 becomes detached from the body of the polype through its whole 



extent, and constitutes the future cell, 

 the subjacent ecderon taking on the 

 ordinary cellular structure. On the 

 pedicle the same process goes on to 

 a less extent, the structureless layer 

 becoming separated only at intervals, 

 so that the pedicle acquires a ringed 

 appearance. 



An integument of one or other of 

 these descriptions is to be met with 

 in all the Sertularian and Actinoid 

 Polypes, and is obviously, in these 

 cases, the result of a process of ex- 

 cretion. In the Mediisce and Beroidm, 

 on the other hand, where the integu- 

 ment is thick and gelatinous, the 

 ecderonic tissue is converted, as a 

 whole, into what closely resembles rudi- 

 mentary connective tissue,in which elastic elements and muscular fibres 

 are developed. The presence of peculiar organs, called the " Thread 

 or Urticating cells" constitutes an extremely characteristic feature 

 in the integument of these creatures. These (yf§-. 311.) are com- 

 posed of a delicate membranous sac («), enclosing a much thicker 

 one {b\ which is open at one extremity, the aperture being stopped 

 by the end of a more or less irregular short stiff sheath (c), some- 

 times giving attachment to several distinct rays or spines {d), 

 applied together, which is fixed to the edges of the aperture, and 

 occupies the axis of the inner sac. To the extremity of this sheath 

 a long, frequently toothed filament is attached {e\ and lies coiled up 

 round the central sheath, and in close contact with the walls of the 



Fig. 311. 



