CHAPTER IX 



THE PHYLUM ANNELIDA 



Segmented worms in which the perivisceral cavity is coelomic ; with 

 a single preoral segment (prostomium) ; with a muscular body wall 

 in which externally the elongated muscle cells are arranged with their 

 longitudinal axes across the width of the worm (circular layer) while 

 internally their axes are parallel to the length of the worm (longitudinal 

 layer) ; with a central nervous system consisting of a pair of preoral 

 ganglia connected by commissures 

 with a pair of ventral cords which 

 usually expand in each segment to 

 form a pair of ganglia from which 

 run nerves to all parts of the seg- 

 ment; with nephridia and coelomo- 

 ducts; and the larva, if present, of 

 the trochosphere type. 



While the above definition is the 

 only one that can be applied to all 

 the annelids, typical representatives 

 of the phylum can also be described 

 as possessing a definite cuticle and 

 bristles or r/?«e^ae composed of chitin, 

 arranged segmentally, imbedded in 

 and secreted by pits of the ectoderm 

 (Fig. 182). The cuticle is thin and 

 not composed of chitin, thus differing 



from that of the Arthropoda. r-u . f r a • 



^ , 111 Fig. 182. Chaeta 01 Lumbricus in 



Four classes compose the phylum. ^^^^ ^^j^ ^^^^^^^ f^^^ Stephen- 



Of these the largest and most typical son. cu. cuticle ; ect. ectoderm ; ch. 

 is that of the Chaetopoda, which are chaeta; cm. circular muscles ; /)r.m. 

 well segmented, have a spacious peri- protractor and n.m. retractor mus- 

 9 , 11 cles : per. peritoneum : joL. follicle 



Visceral coelom and a ways possess ^^^y^,^ formative cdl of chaeta 

 chaetae. All these characters are (^^^^ nucleus), 

 primitive. The Archiannelida is a 



small group characterized by small size, ciliation of skin, loss of external 

 segmentation and often of chaetae. Several members of the group, 

 however, like Saccocirriis, retain chaetae. It is almost certain that the 

 archiannelids are derived from the chaetopods by a process of 

 simplification. The Leeches or Hirudinea are adapted to a specialized 

 mode of life — ectoparasitism — and their whole organization is affected 



