MORPHOLOGY OF INVERTEBRATE TYPES 



of seventeen pairs of transverse branches. Some branches split 

 up into numerous capillary tubes between the different organs, 

 the botryoidal tissue and the hypodermis. These capillary 

 tubes communicate with the capillaries of the sinuses. The 



F . , 



ML 



IB 



FIG. 23. A nephridium of Hirudo medicinalis after Leuckart. 7 1 , tes- 

 tis; VE, vas efferens; VD, vas deferens; LB, lateral blood vessel; AL, apical 

 lobe; TL, testis lobe; F, neptrostome; ML, middle lobe; VDT, vesicle duct; 

 V, ve'sicle. 



dorsal and the ventral sinuses communicate besides with each 

 other in the posterior end of the body. The sinuses with their 

 ramifications represent the codome. They are lined with epithe- 

 lial cells but have no muscular walls. There is no regular blood 

 circulation, but the pulsation of the lateral vessels and their 

 branches propels the blood in the one or the other direction. 



Organs of respiration are absent, the blood being oxidized in 

 the skin. 



The excretory system consists of seventeen pairs of ne- 

 phridia which open to the outside on the ventral surface through 

 as many nephridio pores. The first pair of nephridiopores is 

 situated on the first annulus of the seventh segment, while 



