190 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



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3falacohdella the alimentary canal is a sim- 

 ple tube bent several times upon itself. 

 The alimentary canal is lined by the cells 

 of the endoderm, and the space between 

 them and the ectoderm is occupied by the 

 mesoderm, which contains abundant con- 

 nective and muscular elements, and is ex- 

 cavated by the blood-channels, which some- 

 times have the form of wide sinuses, but 

 in other cases are comparatively narrow 

 vessels with definite walls. 



In the lower Hirudinea^ as Clepsine, 

 the sinuses and vessels appear to form one 

 continuous system of cavities containing a 

 fluid which must be regarded as blood. But 

 in the Leech a distinct pseud-haemal vascu- 

 lar system has attained a great degree of 

 definition and complexity : it consists of 

 (1) a median dorsal trunk ; (2) a median 

 ventral trunk, in which the ganglionic nerve- 

 chain lies ; (3, 4) two wide lateral longitu- 

 dinal trunks (Fig. 50). These anastomose 

 with one another, and give off numerous 

 branches, which open into a rich capillary 

 network, situated in the muscular layer of 

 the mesoderm, and on the segmental and 

 reproductive organs. The fluid contained 

 within these vessels has a red color, and 

 contains no corpuscles. 



More or fewer of the segments of the 

 body are provided with what are termed 

 segmental organs. These are tubes which 

 open externally on the ventral wall of the 

 body, while at their other extremities they 

 either open into the sinuses by ciliated 

 mouths (Clepsine)^ or form a closed and 

 more or less reticulated non-ciliated coil 

 {Hirudo). These obviously answer to the 

 ciliated water-vessels of the Turhellaria 

 and Trematoda. 



The nervous system consists of a cerebral mass in front of 

 the mouth, proceeding from which, on each side, is a commis- 









1 " Die mensclilichen Parasiten," 1863. 



