178 



~-ap.c. 



stm.- 



^pr. 



-^ines. 



B 



\Vi*l, 



ap. 



.mu.sc. 



MARINE WORMS. PHYLUM ANNELIDA 



two pairs of long, slender tentacular cirri, and probably corre- 

 ponds to two fused somites. Behind the last segment is a conica 

 region without parapodia which bears a pair of slender anal 

 ^ cirri and the terminal anus. 



The musculature of Nereis 

 is more complicated than that 

 of the earthworm, the longi- 

 tudinal muscle fibres being 

 grouped into four powerful 

 longitudinal bundles, two dorsal 

 and two ventral, while there 

 are obhque muscles to move 

 the parapodia. As might be 

 expected from the better pro- 

 vision of sense organs on the 

 head, the brain also is more 

 complex. The alimentary canal 

 is simpler than that of the 

 earthworm ; the pharynx can 

 be caused to protrude by being 

 turned inside out, and is lined 

 with cuticle, thickened in 

 places to form numerous small 

 teeth and a pair of strong jaws 

 with which the prey is seized. 

 The sexes are separate. The 

 reproductive organs are very 

 simple, consisting of temporary 

 masses of cells, which arise from 

 the coelomic epithelium. The 

 sexually mature heteronereid 

 forms differ from the less active 

 asexual stage not only in the 

 chctta,' but in having larger eyes and in certain other respects. They 

 swarm near the surface of the sea, and the ova and sperm probably 

 escape through ruptures of the body-wall ; fertilisation takes place 

 in the water. The free young are at first very unlike the parents, 

 being minute, globular creatures, known as trochospheresor trocho- 

 phores (Fig. 127, A), which swim by means of a girdle of cilia in 

 front of the mouth and have an apical tuft at the upper pole. They 

 '— rVrgo a gradual change into the adult, becoming oval and 



seg. 



TTies. 



aTV.Cr 



CLTt. 



Fig. 127. — A, The trochosphere of 

 Nereis. — Modified, after Wilson. 



B. 



an. 



.\ typical trochosphere in an early 

 stage of the transformation into 

 the adult. 



, Anus ; an.c, anal tuft of cilia ; ap.c, apical 

 tuft of cilia ; eye ; m., opening of mouth ; 

 nus., mesoderm ; muse, larval muscles ; 

 nph., larval nephridium ; pr., preoral ring of 

 cilia ; pt., postoral ring ; seg.mes., segments 

 beginning to form in the mesoderm ; sttn., 

 stomodspum (the pouch of ectoderm which 

 forms the mouth and gullet). 



