If present in great numbers on a host, these can be debilitating, and in extreme cases, cause death.Leeches are unusual in that they do not produce certain digestive enzymes such as amylases, lipases or endopeptidases. A deficiency of these enzymes and of B complex vitamins is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hirudo medicinalis, these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, Aeromonas veronii. Non-bloodsucking leeches, such as Erpobdella octoculata, are host to morew bacterial symbionts. In addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut. This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow. Nervous system
A leechs nervous system is formed of a few large nerve cells; their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems. The main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. A nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments six to 26. In segments 27 to 33, other paired ganglia fuse to form the caudal ganglion.
Craniodiaphyseal dysplasia (CDD), also known as lionitis, is an extremely rare autosomal recessive bone disorder that causes calcium to build up in the skull, disfiguring the facial features and reducing life expectancy. These calcium deposits decrease the size of cranial foramina, and can decrease the circumference of the cervical spinal canal. In the few cases recorded, most died in childhood. Cause
The underlying genetics are uncertain. Diagnosis
Among the medical signs are dacryocystitis, seizures, intellectual disability, and paralysis, each of which is a complication resulting from the diminutive foramina. A common sign reported as a result of the disease has been widely spaced eyes. Society
Peter Bogdanovichs 1985 drama film Mask drew public attention to the case of Roy L. "Rocky" Dennis, an American boy who died of the disorder in 1978.In the American medical drama Greys Anatomy episode "Yesterday", Jesse Plemons plays a teenage boy with lionitis. The main character of the two-issue comic book miniseries Friday the 13th: How I Spent My Summer Vacation by Wildstorm Productions is a 13-year-old boy with the disorder. In the anthology television series American Horror Story season 1, Beauregard, the brother of Tate and Adelaide, had lionitis. See also
Leontiasis ossea

References
External links
Craniodiaphyseal dysplasia at orpha.net