396 



ANNELIDA. 



varies very considerably, since, for instance, Branchiobdella astaci, 

 parasitic on the gills of the cray-fish, has but two pairs, while the 

 Gnathobdellidce usually possess seventeen pairs. 



Unicellular glands are present in the Hirudinea in great numbers 

 in the skin and in the deeper layers of the connective tissue. The 

 former secrete a finely granular mucous fluid, which covers the skin ; 

 while the more deeply situated glands, which lie beneath the dermal 

 muscular tunic, secrete a clear viscid substance, which quickly 



hardens outside the body and is used 

 to form the cocoons when the eggs 

 are laid. These glands are espe- 

 cially numerous in the region of the 

 genital openings. 



A blood-vascular system is always 

 present, but in different degrees of 

 development. Portions of the body 

 cavity are transformed into vessel- 

 like trunks, and as a result of this 

 organs which lie in the body cavity 

 teem to be enclosed in blood sinuses. 

 The two lateral vessels and the me- 

 dian blood sinus, which always en- 

 closes the ventral ganglionic chain 

 and sometimes also the alimentary 

 canal (Clepsine, Piscicola), may be 

 interpreted in this manner. In 

 most of the Gnathobdellidce the blood 

 is red, the colour being due to the 

 fluid part of the blood and not to 

 the corpuscles. 



Special respiratory organs are 

 wanting, excepting in Branchellion 

 and some allied leeches, which pos- 

 sess leaf -like branchial appendages. 



The nervous system* in all 

 cases is highly developed. The 

 cerebral ganglia are characterized by a peculiar arrangement of 

 the nerve cells which give rise to swellings on the surface of the 

 ganglia (described by Leydig as a follicular arrangement) (fig. 321). 



* Hermann, " Das Centralneirensystem von Hirudo mcdicinalis," Munchen, 

 1875. 



FIG 321. Anterior end of Hirudo (after 

 Leydig). G, Cerebral ganglion with 

 subcesophageal ganglionic mass ; Sp, 

 sympathetic ; A, eyes ; Sb, sense 

 organs. 



