214 DESCRIPTIONS OF PREPARATIONS. 



varieties, but however much the colouration may change, the form of the 

 teeth distinctive of the medicinal Leech remains constant (Leuckart). 



The somite is not always composed typically of five annuli in Leeches. In 

 Branchellion it has three, in Pontobdella four. The Gnathobdellidae appear to 

 agree with Hirudo. 



The ten-eyed Leeches of Japan possess six segmental papillae on the dorsal, 

 and six on the ventral surface of the annulus that bears them. 



The clitellar somites form the cocoon which contains the ova, a certain number 

 of spermatozoa with albumen, the latter absent in Piscicola. The substance of the 

 cocoon is secreted by the clitellar glands (infra]. When it is fully formed, the 

 animal withdraws its head, and the two ends of the cocoon close up. The openings 

 are plugged by hardened albumen, through which the young Leeches eat their way 

 when ready to escape. Hirudo^ like Aulostoma, lays its cocoon in damp earth. 

 The cocoon is usually attached to some foreign object in the water. Its shape is 

 variable among Leeches. 



The surface of the body is covered by a delicate cuticle perforated by pores, 

 which are the apertures of unicellular glands. This cuticle is continually under- 

 going regeneration, the old one being peeled off, as may be readily seen in a Hirudo 

 kept in confinement. 



The epidermis or hypodermis of the medicinal Leech consists of mallet-shaped 

 cells about j^Vff of an inch long. The heads of the mallets are placed superficially 

 beneath the cuticle. The handles are consequently separated by spaces, into which 

 processes of pigmented connective tissue cells with capillaries make their way. The 

 nucleus lies in the handle of the mallet. The hypodermis cells vary much in 

 character in different Leeches, and in Rhynchobdellidae the pigment cells and capil- 

 laries do not always intrude between them. 



From the hypodermis cells are produced numerous unicellular glands. In 

 Hirudo these glands pass into the dermis, and their ducts are consequently long. 

 Those of the general body-surface appear to be simply mucous glands. The more 

 deeply situated glands are (i) clitellar glands, which occur in the clitellar region in 

 groups of four to five, the glandular part lying in the longitudinal muscle layer ; 

 (2) prostomial glands of doubtful function, but probably found only in Gnathobdel- 

 lidae, with ducts opening round the edges of the buccal cavity, and their contents 

 not staining with borax carmine; (3) salivary glands, which belong to the buccal 

 cavity itself, with ducts opening on the ridges which bear the teeth, and with 

 contents which stain with borax carmine. 



The hypodermis cells are also modified to form sensory cells, with which 

 nerves are continuous. In the head region are found the goblet-like bodies and 

 the eyes, the visual nature of which is doubted by Carriere. Of the latter there are 

 ten, two on the first and second annulus, and two on the first annulus of the three 

 following somites. Each eye consists of a projecting cap of short non-pigmented 

 hypodermis cells, covered by cuticle and containing at their outer ends small re- 

 fractile bodies : of an external layer of pigmented cells, separated by a lamina (?) 

 from a layer of nucleated clear cells with well-defined walls and protoplasm largely 

 replaced by a vacuole, and lodging a corpuscle of unknown significance : and of a 



