202 SEGMENTED WORMS OR ANNELIDA. 



tide comes in, tlie mouth may protrude. The animal is 

 able to turn in its burrow. 



The young are pelagic, and it is said by some that the 

 adults are active and swim about at certain seasons. 



External Appearance. 



The lob worm varies in length from eight inches to a 

 foot, and at its thickest part is about half an inch in 

 diameter. There are three regions in the body : — (a) the 

 anterior seven segments, of which all but the first have 

 bristles ; ib) the gill bearing region of thirteen segments ; 

 {c) the thinner posterior part of variable length, without either 



Fin. 64. — Arenicola piscatoriira. (After CuxMM'.HAM 

 and Ramage.) 



Note anterior protrusion of mouth ; seta; on anterior region ; 

 seta; and gills on median region ; thinner tail region often longer 

 than shown. 



bristles or gills. The head lobe is very small ; there are no 

 tentacles or eyes. Anteriorly a soft proboscis is protruded. 



Skin and Muscles. 



Each segment is marked by several rings ; there are 

 numerous warts on the posterior region. IMost externally 

 lies the cuticle, then the pigmented epidermis, then the 

 circular and the longitudinal muscle fibres. 



Appendages. 



Unlike many of the marine Annelids which have on each 

 segment well-developed outgrowths or parapodia, divided 

 into a dorsal notopodium and a ventral neuropodium, 

 Arenicola has very rudimentary appendages. This reduc- 

 tion of appendages must be associated with the animal's 

 mode of life ; the same is true of many tube inhabiting 

 worms. The first segment has no trace of appendages, 



