22 September

Topic One--The pseudocoelomates
End-point in invertebrate evolution rather than a connecting link
Three features:

1. constant cell number
2. growth takes place by increases in cell size rather than cell
3. number cuticle is non-living

Topic Two--Relationship between embryology and phylogeny

Ontogeny recapitulates phylogeny (Ernst Haeckel)--patterns of embryologic development show the patterns of organismal relationships

Three important aspects

1. pattern of mouth and anus formation
   
2. way in which the mesoderm forms
   
3. the cell lineage is determined (at what stage and in what manner)

Two main groups, the protostomates and the deuterostomates. both groups are true coelomates (eucoelomates)

protostomates--the first opening is the mouth,  the coelom forms by schizocoely and development is determined at an early stage-the embryo shows spiral cleavage. coelom is often called a haemocoel (continuity between coelom and vascular spaces)

deuterostomes--first opening is the anus, the coelom forms by enterocoely and development is not determined at an early stage.  the embryo shows radial cleavage and typically there is a separation between vascular and coelomic spaces.