388 PHYSIOLOGY OF THE INVERTEBRATA. 



swim, though its motion in water is rather slow. " Whilst 

 swimming, the body becomes flattened by the contraction of 

 the vertical muscle-fibres, which pass from the dorsal to the 

 ventral surface ; and then by perpendicular quick serpentine 

 undulations, it progresses like a wavy ribbon." 



The muscle-fibres are developed, as in the higher animals, 

 out of nucleated, spindle-shaped, muscle-cells. The fibres 

 are not transversely striated, but they are enveloped in a 

 structureless sheath. 



In the OligocTimta, represented by Lurribricus, the muscular 

 system is somewhat similar to that already described. 

 Beneath the cuticle and hypodermis there is an external 

 layer of circular muscle-fibres and an internal one of longi- 

 tudinal muscle-fibres ; there are also radiating and obliquely 

 intertwisted fibres. On the ventral surface of each somite 

 (in Lumhricus) four pairs of minute pits occur, from each of 

 which projects a long hook-like seta or bristle. The setse or 

 bristles can be projected or retracted at will, and they aid 

 locomotion in a somewhat similar manner to the suckers of 

 Hirudo. Both these devices are the means of anchorage,, 

 while the subcutaneous muscles produce the vermicular 

 motions of the body. 



The Polychceta, or marine worms, are usually provided with 

 parapodia (rudimentary limbs), having numerous chitinous 

 setae embedded in them. In the body-wall the circular and 

 longitudinal muscle-fibres are well developed. The sub- 

 order Errantia contains the free-swimming Polyclicata. The 

 head of these animals contains tentacula, and generally 

 cirri, and the anterior portion of the pharynx is like a pro- 

 boscis, being eversible. The parapodia are well developed in 

 the Errantia. 



The Tiibicola, or sedentary Polychceta, have no cirri, and 

 the parapodia are only slightly developed. None have a 

 proboscis or eversible pharynx. The TuUcola are not free 

 and actively locomotive animals like the Ero^antia. They 

 live in tubes, which they construct either by gluing together 



