III. ANNELIDA: CtL-ETOPODA 



273 



organs thus becoming sexual ducts. The second of these conditions explains 

 the coexistence of reproductive funnels and nephridia in the genital segments 

 of many oligochastes. In the oligochaites there are other modifications: Opening 

 of the nephridia into fore or hind gut, connection of the tubules into a network 

 with several openings to the exterior in each segment (Megascolicida,-). 



In many marine annelids there occurs a metamorphosis in which 

 pelagic larvae occur. These, in spite of many modifications, are com- 

 parable with 'Loven's larva,' the trochophore already described (p. 238). 



II. 



FIG. 256. Different relations of nephridia and sexual ducts in chastopods (after 

 Goodrich). I, hypothetical primitive condition; II, Phyllodoceids and Goniads; III, 

 Dasybranchus; IV, Syllids, Spionids, etc. In I and III the ciliated grooves (sexual 

 sacs, g) lead the sexual products (ei) direct to the exterior; in II and IV they empty 

 into the nephridial canals, which are either protonephridia (I and II) covered with 

 solenocytes, or (III and IV) are nephridia. 



The differences largely consist of modifications of the ciliary apparatus; 

 the number of bands may be increased (polytroclie larvae), or a single band 

 may occur at the middle (mesotroche) or at the end (telotroche) of the body. 

 The larva becomes a segmented worm by the hinder end of the larva 

 growing out and dividing into segments (fig. 257, B}. In this growth new 

 mesoderm develops as a pair of bands (usually from a pair of cells at the 

 hinder end, the teloblasts). This mesoderm divides, from in front back- 

 ward, into the primitive segments. Each of these become hollow, forming 

 a coelomic chamber. Since these, right and left, grow around the diges- 

 tive tract, they give rise to the somatic and splanchnic mesothelium and 

 form part of the digestive and body walls. Where they come in contact 

 above and below the intestine they form the mesenteries which frequently 

 is 



