SEEPULA CONTOETUPLICATA. 



265 



manent residence that encloses and defends them. This is generally an 

 elongated tube, varying in texture in different species. Sometimes it 

 is formed by agglutinating foreign substances, such as grains of sand, 

 small shells, or fragments 

 of various materials, by 

 means of a secretion that 

 exudes from the surface 

 of the body and hardens 

 into a tough membranous 

 substance ; such is the 

 case of Terebella Medusa 

 (fig. 132). In other cases, 

 as in the Serpula contor- 

 tuplicata (fig. 131), the 

 tube is homogeneous in 

 its texture, formed of 

 calcareous matter resem- 

 bling the shells of certain 

 bivalve mollusca, and ap- 

 parently secreted in a 

 similar manner. These 

 tubes are generally found 

 incrusting the surface of 

 stones or other bodies that 

 have been immersed for 

 any length of time at the 

 bottom of the sea ; they are closed at one end, and from the oppo- 

 site extremity the head of the worm is occasionally protruded in 

 search of nourishment. It must be evident that, in animals thus en- 

 cased, the character of the respiratory apparatus must be considerably 

 modified*; instead, therefore, of the numerous branchiaB appended to 



* M. de Quatrefages gives the following resume of the various modifications met 

 with in the respiratory apparatus of the Annelidans : 



' 1. Eespiration general and entirely cutaneous (Lwnbriconereis). 



' 2. Eespiration cutaneous, but confined to certain segments ( Cfuetopterus). 



' 3. Eespiration cutaneous, but confined to certain points of each segment (Nereis). 



' 4. Eespiratory organs taking the form of a simple caecum or bladder ( Grlycera}. 



' 5. The branchiae characterized more and more by the formation of a canal in 

 connexion with larger or smaller lacunae. 



' 6. These branchiae may be distributed all along the body (Eunice sanguined}. 



' 7. They may be confined to a certain number of segments placed towards the 

 in ddle of the body (Arenicola, Hermella). 



" 8. They may all be placed at the extremity of the body, so as to form a double 

 tuft." 



" The breathing is accomplished," says Dr. Williams, " in every species, the Earth- 

 worm not excepted, in strict conformity with the aquatic principle. No known Anne- 

 lid respires on the atmospheric model. In every Annelid the blood, though variable 



Serpula contortuplicata. 



