Comparative Vertebrate Anatomy, 2nd hour exam
Nov. 18, 2002
1. Outline the changes and reasons for them in the evolution of the aortic arches
(amphioxus to mammals). (25)
Start with many paired arches connecting ventral to dorsal aorta through
pharynx=Amphioxus
In sharks – 7 pair, broken into capillary bed in gills – ventral to dorsal aorta
In higher fish down to 4 arches, first three present in embryo but lost in head circulation
in adult
In Amphibia – start with three paired arches. From ventral aorta to dorsal aorta.
One goes to lungs – but there is a lung bypass (present in all embryos but also in adult
here)
Two go to body, one bigger than the other.
In Reptiles we see that the ventral aorta is gone. It has “split” in such a way that the
arches come directly off the heart. This is to compliment the development of the lung
circulation and the “parallel” arrangement of the heart, with one side serving the lungs
with unoxygenated blood from body and the other other side serving the body with
oxygenated blood from the lungs.
By advanced reptile state there are two arches – a pair to the lungs and a pair to the body.
In Birds and mammals there are still a pair of arches to the lungs but only one arch,
coming off the heart and going to the body. However, in birds and mammals this arch
goes to the body on different sides of the heart, crossing over in reptiles and not in
mammals. Since you can’t derive one pattern from the other, it indicates that in primitive
reptiles the ventral aorta split back to the heart in two different ways in the ancestors of
mammals and of most birds and reptiles.
2. Outline the changes, and reasons for them, in the evolution of methods of getting
sperm from the male testes to the female (amphioxus to mammals). (25)
Two parts to essay. One is getting sperm from testes to almost out of body, and the other
is how to get sperm into the female.
a) start with sperm released into abdominal cavity, swimming through cavity and
exiting through an abdominal pore . this is in lamprey
in Shark you see a connection from testes to the archinephric duct- the duct
serving the kidney, and the sperm share this duct to get to outside.
In teleosts the sharing is eliminated by forming a new duct for the testes.
In Amphibia, reptiles, mammals, etc. – the sharing is eliminated by forming a new
duct and a new kidney structure and the testes retaining the archinephric duct as the
pathway to the outside.
b) In most fish, amphibians and birds there are no special devices to get sperm into the
femaile. In fish and amphibian the sperm are spewed outside for external
fertilization. In birds the cloaca is everted to transfer the sperm into the female’s
cloaca for internal fertilization. In sharks, claspers-projections on the pelvic fins
provide a path for sperm to follow for internal fertilization. In reptiles there are
hemipeni which serve the same purpose. In mammals and turtles there is a penis –
a single projection which sperm can swim on or through. Penis shape is roughly
comparable to vagina shape in mammals – y shaped if a double vagina, single if a
single vagina.
c)
3. For each of the following, explain its origin, purpose for existence, and or special
abilities. (10 each)
a. mesanephric kidney
the embryonic kidney in reptiles, birds and mammals which is non
functional as a kidney but serves as an organizer to lay down the
archinephric duct, which will become the tube system for the gonad.
b. air bladder
a derivative of the lung. In teleosts paired outpocketings of the pharynx
start as a lung would, but loose their connection to the pharynx and fuse
dorsally, forming an internal sac. By regulating the amount of gas
(nitrogen) in the sack, fish can maintain hydrostatic balance in the ocean.
c. azygous vein
an unpaired vein along the back of mammals that receives blood from the
trunk somites (ribs, muscles, etc) and dumps into the anterior vena cava
(precava). It is a remnant of the original posterior cardinal vein, which
appears in the early embryo of mammals but which is replaced by the vena
cava as the main channel for blood return to the heart as development
changes the heart and circulation from a bilateral system to an unpaired
asymmetric system.
.I
d. gizzard in birds
the bird stomach, possessing a muscular wall which, with the aide of
stones swallowed by the bird, allows grinding of hard substances like
seeds.
e. lung in birds
similar to all other lungs in its origin (out pocketing of the pharynx ) but
unique in that it is a flow-though system rather than a one way pocket
system. Flow through is possible because of air sacs anterior and posterior
to the lungs. The one way flow of air through the lungs allows flowcounterflow exchange with the blood stream and is very efficient,
allowing the lung to be reduced in size and weight – good for birds that
need a light body to fly.