266 MANUAL OF ZOOLOGY. 



secreted. The young Tubicolar Annelide thus resembles the 

 permanent condition of the Errant forms ; and the stationary 



Fig. 131. Development of Tubicolar and Errant Annelides A, Larva of Tercbella; 

 o Position of the mouth ; a Anus, surrounded by the posterior circlet of cilia; c An- 

 terior circlet of cilia ; t Tentacle. B, Polytrochal larva of Arenicola; C, Larva of 

 Pnyllodoce. D, Larva of Spirorbis ; tt Tentacles. All the figures are greatly 

 magnified. (After Claparede, Schultze, and A. Agassiz.) 



condition of the adult, accompanied by the loss of its sense- 

 organs, may be regarded as an instance of "retrograde de- 

 velopment." 



The most familiar of the Tubicola is the Serpula (fig. 130, a), the con- 

 torted and winding calcareous tubes of which must be known to almost 

 every one as occurring on shells or stones on the sea-shore. One of the 

 cephalic cirrhi in Serpula is much developed, and carries at its extremity 

 a conical plug, or operculum, whereby the mouth of the tube is closed 

 when the animal is retracted within it. The operculum of Serpula has a 

 more than ordinary interest in the fact that it is the only instance in the 

 Annelida in which calcareous matter is deposited within the integument. 

 In Spirorbis (fig. 130, b) the shelly tube is coiled into a flat spiral, one 

 side of which is fixed to some solid object. It is of extremely common 

 occurrence on the fronds of sea- weed and on other submarine objects. 



Equally familiar with Serpula is Terebella, the animal of which is in- 

 cluded in a tube composed of sand and fragments of shell, cemented together 

 by a glutinous secretion. In the Sabellida the tube is composed of granules 

 of sand or mud. In Pectinaria the tube is free, membranous, or papyra- 

 ceous, covered with sand-grains, and in the form of a reversed cone of con- 

 siderable length. In Phoronis, the tube is membranous, and the branchiae 

 are carried upon a horse-shoe-shaped process, which is strikingly similar to 

 the " lophophore " of the Polyzoa, 



ORDER IV. ERRANTIA ( Chcetopoda, or Nereided], This order 



