mammals and the terrestrial and semi-terrestrial
amphibians. The situation of the reptiles is interesting but, unfortunately, inadequately investigated. The soft-shelled eggs of many snakes and
turtles and also the hard-shelled eggs of the crocodile are permeable, and the developing
embryos in these eggs always produce urea,
although the adult animals are uricotelic. Surprisingly, in the eggs of the turtle Trionyx spiniferus,
70 % of the excreted nitrogen is urea, 24 %
ammonia and only 6 % uric acid, despite the fact
that these eggs are completely impermeable
[176]. Owing to special adaptations, the elasmobranchs and crossopterygians are also ureotelic,
as are the lungfish and terrestrial oligochaetes
under certain conditions. In fact, animal excretions usually contain all three substances in various proportions, as well as further nitrogenous
compounds, such as amino acids, allantoin or
allantoic acid, trimethylamine oxide and other
amines, creatine, creatinine, and nitrogenous
substances that are partly of exogenous origin.
The proportions of these substances greatly
depend upon internal and external conditions.
The Platyhelminthes are primarily ammoniotelic. However, the terrestrial turbellarian Bipalium kewense, the endoparasitic trematodes Fasciola hepatica and Schistosoma mansoni, and the
cestode Hymenolepsis diminuta produce and
excrete substantial amounts of urea. Similarly,
amongst the ammoniotelic Nemathelminthes several species excrete up to 20 % urea. Ascaris
lumbricoides maintained in a small amount of
external medium significantly increases its output
of urea. All investigated nematodes excrete about
20-30 % of their nitrogenous waste as amino
acids; Trichinella spiralis larvae and Ditylenchus
sp. additionally produce small amounts of amines
[18, 82].
Cephalopods and bivalves are strictly ammoniotelic. Pelagic squids of the family Cranchiidae,
such as Helicocranchia pfefferi, make special use
of their ammoniotelic nitrogen metabolism. A
coelom sac, which takes up most of the mantle
cavity, is filled with a fluid containing about
480 mmolll ammonium chloride. Owing to its
lower density relative to seawater, this fluid exerts
a buoyancy that allows the animal to float.
Cephalopod urine also contains guanine, and that
of the mussels contains 25-38 % of the nitrogen
in the form of amino acids, with an additional
7-13 % urea in the case of Mya and Crassostrea
[26,82].
In parallel with the great variety of their habitats, the gastropods show a wide spectrum of
12.3 End Products of Nitrogen Metabolism
411
nitrogenous excretions. Aquatic snails excrete
mainly ammonia, but in addition excrete speciesand season-dependent amounts of urea. Uric acid
is stored for a long time in the snails, and very
variable results are obtained from analysis of the
nephridium content and the excretions. The
amounts of uric acid stored in aquatic forms are
often just as high as in terrestrial varieties. Terrestrial gastropods produce urea and uric acid, and
part of the latter is retained for long periods.
Members of the families Helicidae and Limacidae
also produce guanine and xanthine. Urea production depends upon the species and the conditions,
e.g. Helix pomatia appears to produce only uric
acid, whereas in Lymnaea urea makes up about
14 %, and in Biomphalaria in some situations it
constitutes more than 50 % of the excreted nitrogen [26, 82]. The aquatic pulmonate snail Biomphalaria glabrata is the intermediate host for the
trematode Schistosoma mansoni, the agent for
bilharziosis. Increased protein degradation following the withdrawal of nutrients or infection
with Schistosoma larvae causes an increase in the
urea fraction from 20 to 55-60 % in Biomphalaria
excretions. Under normal conditions terrestrial
snails experience dramatic de- and rehydration
events, but the resulting alterations in the osmolality of the extra- and intracellular fluids are
tolerated. In the dry resting phase, several species
markedly increase their synthesis and storage of
urea; examples include Bulimulus dealbatus,
Limax flavus, Strophocheilus oblongus and
Anguispira alternata. The urea concentration in
the haemolymph may increase to over 400 mm01l1.
In other species, e.g. Olata lactea and Helix
aspersa, urea is synthesized in large amounts but
is continuously hydrolysed by a highly active
urease; these species therefore excrete gaseous
ammonia.
Amongst the annelids, polychaetes and leeches
are all ammoniotelic. Amino acids are excreted
by some leeches and oligochaetes, and urea
excretion occurs in starving oligochaetes. The
proportion of urea in the excretions and its
increase with hunger varies with the species; in
Eisenia foetida it is always less than 22 % and
increases little with hunger, whereas in Lumbricus terrestris and Allolobophora calliginosa urea
constitutes up to 90 % of the excreted nitrogen.
The situation is similar in the freshwater oligochaetes. In Lumbricus rubellus, urea excretion
increases with hunger, although ammonia is still
excreted in large amounts. In contrast, Eisenoides
lonnbergi remains purely ammoniotelic. The
conditions bringing about the switch to ureotelic
amphibians. The situation of the reptiles is interesting but, unfortunately, inadequately investigated. The soft-shelled eggs of many snakes and
turtles and also the hard-shelled eggs of the crocodile are permeable, and the developing
embryos in these eggs always produce urea,
although the adult animals are uricotelic. Surprisingly, in the eggs of the turtle Trionyx spiniferus,
70 % of the excreted nitrogen is urea, 24 %
ammonia and only 6 % uric acid, despite the fact
that these eggs are completely impermeable
[176]. Owing to special adaptations, the elasmobranchs and crossopterygians are also ureotelic,
as are the lungfish and terrestrial oligochaetes
under certain conditions. In fact, animal excretions usually contain all three substances in various proportions, as well as further nitrogenous
compounds, such as amino acids, allantoin or
allantoic acid, trimethylamine oxide and other
amines, creatine, creatinine, and nitrogenous
substances that are partly of exogenous origin.
The proportions of these substances greatly
depend upon internal and external conditions.
The Platyhelminthes are primarily ammoniotelic. However, the terrestrial turbellarian Bipalium kewense, the endoparasitic trematodes Fasciola hepatica and Schistosoma mansoni, and the
cestode Hymenolepsis diminuta produce and
excrete substantial amounts of urea. Similarly,
amongst the ammoniotelic Nemathelminthes several species excrete up to 20 % urea. Ascaris
lumbricoides maintained in a small amount of
external medium significantly increases its output
of urea. All investigated nematodes excrete about
20-30 % of their nitrogenous waste as amino
acids; Trichinella spiralis larvae and Ditylenchus
sp. additionally produce small amounts of amines
[18, 82].
Cephalopods and bivalves are strictly ammoniotelic. Pelagic squids of the family Cranchiidae,
such as Helicocranchia pfefferi, make special use
of their ammoniotelic nitrogen metabolism. A
coelom sac, which takes up most of the mantle
cavity, is filled with a fluid containing about
480 mmolll ammonium chloride. Owing to its
lower density relative to seawater, this fluid exerts
a buoyancy that allows the animal to float.
Cephalopod urine also contains guanine, and that
of the mussels contains 25-38 % of the nitrogen
in the form of amino acids, with an additional
7-13 % urea in the case of Mya and Crassostrea
[26,82].
In parallel with the great variety of their habitats, the gastropods show a wide spectrum of
12.3 End Products of Nitrogen Metabolism
411
nitrogenous excretions. Aquatic snails excrete
mainly ammonia, but in addition excrete speciesand season-dependent amounts of urea. Uric acid
is stored for a long time in the snails, and very
variable results are obtained from analysis of the
nephridium content and the excretions. The
amounts of uric acid stored in aquatic forms are
often just as high as in terrestrial varieties. Terrestrial gastropods produce urea and uric acid, and
part of the latter is retained for long periods.
Members of the families Helicidae and Limacidae
also produce guanine and xanthine. Urea production depends upon the species and the conditions,
e.g. Helix pomatia appears to produce only uric
acid, whereas in Lymnaea urea makes up about
14 %, and in Biomphalaria in some situations it
constitutes more than 50 % of the excreted nitrogen [26, 82]. The aquatic pulmonate snail Biomphalaria glabrata is the intermediate host for the
trematode Schistosoma mansoni, the agent for
bilharziosis. Increased protein degradation following the withdrawal of nutrients or infection
with Schistosoma larvae causes an increase in the
urea fraction from 20 to 55-60 % in Biomphalaria
excretions. Under normal conditions terrestrial
snails experience dramatic de- and rehydration
events, but the resulting alterations in the osmolality of the extra- and intracellular fluids are
tolerated. In the dry resting phase, several species
markedly increase their synthesis and storage of
urea; examples include Bulimulus dealbatus,
Limax flavus, Strophocheilus oblongus and
Anguispira alternata. The urea concentration in
the haemolymph may increase to over 400 mm01l1.
In other species, e.g. Olata lactea and Helix
aspersa, urea is synthesized in large amounts but
is continuously hydrolysed by a highly active
urease; these species therefore excrete gaseous
ammonia.
Amongst the annelids, polychaetes and leeches
are all ammoniotelic. Amino acids are excreted
by some leeches and oligochaetes, and urea
excretion occurs in starving oligochaetes. The
proportion of urea in the excretions and its
increase with hunger varies with the species; in
Eisenia foetida it is always less than 22 % and
increases little with hunger, whereas in Lumbricus terrestris and Allolobophora calliginosa urea
constitutes up to 90 % of the excreted nitrogen.
The situation is similar in the freshwater oligochaetes. In Lumbricus rubellus, urea excretion
increases with hunger, although ammonia is still
excreted in large amounts. In contrast, Eisenoides
lonnbergi remains purely ammoniotelic. The
conditions bringing about the switch to ureotelic