The three major clades of bilaterian animals are the

The three major clades of bilaterian animals are the ecdysozoa, lophotrochozoa, and deiterstomia. 
Ecdysozoa are organisms that include insects, crustaceans, chelicerata, and myriapods. Lophoytochozoa include fossil organisms. Deiterstomia include chordata, echinodermata, and hemichordata.
Bilaterian animals have bilateral symmetry, meaning that they have a head, and a tail, as well as back and belly. 

a. Nucleolus: The nucleolus refers to a composition, which develops with the assistance of the nucleolar organizer region present in the eukaryotes. It constitutes protein and ribosomal DNA and is a location where the generation of ribosomes takes place.  

b. Centromere: The prime function of the centromere is to function as the point of attachment for sister chromatids and is a place where the attachment of chromosomes and spindle fibers takes place. The differentiation of centromere takes place at the time of mitosis and meiosis II, which helps in the distribution of chromosomal substances to the daughter cells.  

c. Ribosome: The ribosome refers to the composition where different forms of RNAs, enzymes and other components help in the alignment of the primary sequence of a specific protein.  

d. Chromatin: Chromatin constitutes the genetic information, which helps in sustaining hereditary information and generation of a phenotype.  

e. Centriole: The centriole refers to a cytoplasmic composition, which via the help of spindle fibers assists in the migration of chromosomes at the time of meiosis and mitosis, mainly in the animal cells.  

f. Mitochondria: The mitochondrion refers to a membrane-bound composition situated in the eukaryotic cytoplasm. It is the place where the production of huge amounts of ATP takes place via the process of oxidative phosphorylation.