366 ANNELIDA. 



almost all cases a visceral nervous system (sympathetic). The 



following sense organs are found : paired eye s])ots with refractive 

 structures, or larger more complicated eyes; also auditory vesicles 

 upon the cesophageal ring (branchiate worms), and tactile organs. 

 The latter have, in the Chcetopoda, the form of tentacles and tentacular 

 cirri on the head and of cirri on the parapodia. When tentacles 

 and cirri are absent, the anterior end of the body and the region of 

 the mouth seem to function as tactile organ.-,. 



Vascular system. A blood vascular system is very commonly 

 present ; in many cases, however, it seems not to be completely 

 closed, but to communicate with the body cavity, which contains 

 blood. Two main vascular trunks, a dorsal and a ventral, connected 

 with one another by numerous transverse anastomoses, are generally 

 present. The blood is usually coloured (green or red), and its cir- 

 culation is effected by the contractility of the walls of certain vessels ; 

 sometimes the dorsal vessel, sometimes the ventral, and sometimes 

 the transverse connecting vessels are contractile. Lateral longi- 

 tudinal vessels are often present in addition to the above. In the 

 Hirudinea these, as well as the median contractile blood sinus, are 

 probably to be regarded as isolated parts of the body cavity. 



Special respiratory organs are found amongst the Chcetopoda in 

 the branchiate worms. 



The excretory organs, corresponding to the water-vascular or 

 excretory system of the Platyhelmintb.es, have the form of coiled 

 canals (segmental organs or nephridia), which are repeated in pairs 

 in each segment. Each nephridium usually begins with a ciliated, 

 funnel-shaped opening into the body cavity, and opens to the exterior 

 by a lateral pore (fig. 70). These may assume in certain segments 

 the function of generative ducts, e.g., the nephridia of the Gephyrea, 

 which, however, are much reduced 'n number. In the cephalic 

 segment or head there is also a segmental organ (head kidney), 

 which in the larva functions as a kidney and later disappears. 



Reproduction. Considering the independence of the segments, to 

 which we ascribe the value of a subordinate (morphological) indi- 

 viduality, the occurrence of asexual reproduction by fission and 

 gemmation in the long axis (Chcetopoda) is not surprising. Nume- 

 rous Annelida (Oligochceta, Hirudinea) are hermaphrodite ; the 

 marine ChcKtopoda, on the contrary, are for the most part of separate 

 sexes. Many lay their eggs in special sacs and cocoons, in which 

 case development is direct, without metamorphosis. The marina 

 worms, on the contrary, undergo a more or less complicated 



