8. Pseudo-metamerism/strobilization:
1. Segmentation of the body is related to the segmentation of the ectoderm.
2. New segments are formed at the anterior part of the body (behind the
scolex).
3. Segments work as an independent unit.
4. No co-ordination among the segments e.g., tapeworms (Platyhelminthes).
Metamerism:
1. Segmentation of the body is related to the segmentation of the
mesoderm.
2. New segments are formed at the posterior end of the body (in front of the
anal segment).
3. Segments work as different units.
4. Segments work in co-ordination with all other segments e.g., Annelida,
Arthropoda, Chordata.
Complete Metamerism:
When the segmentation is seen practically in all systems, the metamerism is
called complete metamerism. It is seen in annelids.
Incomplete Metamerism:
When the segmentation is not seen in all the organs, the metamerism is
called incomplete metamerism. It is seen in arthropods and chordates.
23. Theria:
1. Eggs or Young ones:
Viviparous
2. Mammary glands:
Mammary glands with nipples or teats.
3. Pinna:
Generally present (absent in aquatic mammals).
4. Beak:
Mammary glands with nipples or teats.
5. Cloaca or anus:
All therians possess anus expected pikas (pika), a lagomorph
type.
6. Teeth:
Present except scaly ant-eater (Man is sp.) and Baleen whale
etc.
7. Position of testes:
Generally within the scrotal sacs.
8. Ureters:
The ureters open directly into the urinary bladder.
(a) Coracoid small
(b) Scapula with a spine
9. Pectoral girdle:
(a) Coracoid small
(b) Scapula with a spine
10. Interclavicle:
Absent
11. Fully homeothermic or not:
Fully homeothermic
Example:
Kangaroo (Macropus giganteus)
Indian elephant (Elephas maximus)
Man (Homo sapiens sapiens)
Prototheria:
1. Eggs or Young ones :
Oviparous
2. Mammary:
Mammary glands are without nipples or teats.
3. Glands:
Indistinct or absent.
4. Pinna:
Present and is formed by the nasal and premaxilla
bones.
5. Beak Cloaca or anus:
Cloaca
6. Teeth:
Absent, only temporary molar milk teeth present in
Ornithorhynchus which disappear in adult state.
Tachyglossus lacks teeth in any stage of development.
7. Position of testes:
Abdominal.
8. Ureters:
The ureters open into a urinogenital sinus, not open into
the urinary bladder.
9. Pectoral girdle:
(a) With a large coracoid
(b) Scapula without spine
10. Interclavicle:
T-shaped interclavicle
11. Fully homeothermic or not:
Not fully homeothermic
Example:
Platypus (Ornithorhynchus sp.)
