194 THE ANATOMY OF INVERTEBRATED ANIMALS. 



In the majority of the segments there are, as in the Hi- 

 rudinea, paired segmental organs ; these are ciliated and 

 their inner ends open into the perivisceral chamber. 



The nervous system consists of prae-oral or cerebral gan- 

 glia, continued backward, on the ventral aspect of the body, 

 by commissures on each side of the oesophagus into a double 

 chain of closely united post-oral ganglia. 



Large tubular fibres are imbedded in the neurilemma of 

 the ganglionic chain on its dorsal face. In the earthworm 

 there are three of these — one median and two lateral — ex- 

 tending along the whole length of the ventral end, but not 

 into the oesophageal commissures. 1 The nature of these 

 structures is unknown. 



These animals are hermaphrodite. The generative organs 

 are situated in the front part of the body, the male organs 

 being anterior to the female. In the aquatic Oligochceta 

 (JYcds, Tubifex) the genital glands have no proper ducts, but 

 the segmental organs of the segments in which they are con- 

 tained convey the generative products outward. In the ter- 

 ricolous forms (Lumbricus) the vasa deferentia are continuous 

 with the testes, which are very large. The ovaries, on the 

 other hand, are minute solid bodies attached to one of the 

 mesenteries, and the oviducts are separate tubes with funnel- 

 shaped mouths, which open into the cavity of the segment. 



In JSfais and Chcetogaster, agamic multiplication occurs 

 by the development of posterior segments of the body into 

 zooids, which may remain associated in chains for some time, 

 but eventually become detached and assume the parental 

 form. Schulze has observed that when a JVais has divided 

 into an anterior and posterior zoftid, the last somite of the 

 former gradually enlarges, and becomes divided into new 

 somites, the anterior of which give rise to a head. A new 

 zooid is thus developed between the previously existing ones. 

 This process is repeated in what was the penultimate, but is 

 now the ultimate somite of the anterior zooid ; and again in 

 the penultimate somite when it has, in the same way, become 

 terminal. 



As the Earthworm is a very accessible subject, it may be 

 useful to the student to be furnished with an account of some 

 of the chief points of its organization more in detail. 



The exterior of the body of an Earthworm (Lumbricus 

 terrestris, rubellus, or communis) shows a number of close-set 



1 Claparede, " Histologische Untersuchungen iiber den Regenwurm," 1869. 



