. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 396 ANNELIDA. varies very considerably, since, for instance, BrancMobdella astaci, parasitic on the gills of the cray-fish, has but two pairs, while the Gf-nathobdellidce 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, secre
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 396 ANNELIDA. varies very considerably, since, for instance, BrancMobdella astaci, parasitic on the gills of the cray-fish, has but two pairs, while the Gf-nathobdellidce 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 s-eem 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, Piscicolci), 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 Ley dig as a follicular arrangement) (fig. 321). * Hermann, " Das Centralncrvcnsystern von Hirudo medicinalis," Miinchen, FIG 321.—Anterior end of Hirudo (after Leydig). G, Cerebral ganglion with subcesophageal ganglionic mass ; Sp, sympathet
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