CHAPTER IX 



THE PHYLUM ANNELIDA 



^-ect. 



Segmented worms in which the perivisceral cavity is coelomic ; with 

 a single preoral segment (prostomium) ; 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; 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 chaetae com- 

 posed of chitin, arranged segmen- 

 tally, imbedded in and secreted by 

 pits of the ectoderm (Fig. 172). The 

 cuticle is thin and not composed of 

 chitin, thus differing from that of 

 the Arthropoda. 



Four classes compose the phylum. 

 Of these the largest and most typical 

 is that of the Chaetopoda, which are 

 well segmented , have a spacious peri- 

 visceral coelom and always possess 

 chaetae. All these characters are 

 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, how- 

 ever, like Saccocirrus, retain chaetae. 

 It is almost certain that the archian- p^g 1^2. Chaeta of Lumbricus in 

 nelids are derived from the chaeto- body wall. Altered from Stephen- 

 pods by a process of simplification, son. cu. cuticle; ecu ectoderm; ch. 

 U^, T 1 TT- J- chaeta I c.7«. circular muscles ; ^r.w. 



The Leeches or Hirudinea are p^^^^^^',^, .^^ run. retractor mus- 

 adapted to a specialized mode of life ^^^^ . p^y peritoneum ; jol. follicle 

 — ectoparasitism — and their whole and fm.c. formative cell of chaeta 

 organization is affected by it. They (with nucleus), 

 retain the segmentation characteristic of the phylum in most of their 

 organs but the coelom is usually much restricted and broken up into 

 a system of small spaces and the chaetae are lost. In one primitive 

 form, Acanthobdella, there are chaetae and a spacious perivisceral 

 coelom in the anterior segments. In all leeches the anterior and 



