In this article we will discuss about the affinities of branchiostoma.
Affinities # Branchiostoma with Mollusca:
Molluscs and Branchiostoma differ widely. Superficial resemblances in ciliary feeding and respiratory mechanism (no gill slits in molluscs) have no phylogenetic significance.
Affinities # Branchiostoma with Echinodermata:
Similarities:
1. Early development cleavage, gastrulation and mesoderm formation.
ADVERTISEMENTS:
2. Three separate enterocoelic cavities.
3. The pores of the calyx of some fossil carpoid echinoderms are similar to the gill slits of Branchiostoma.
4. Presence of creatin phosphate and phosphagens.
At present, the echinoderms are not considered as an ancestor of chordates. The similarity is presumably due to a remote common origin of both the groups.
Affinities # Branchiostoma with Hemichordata:
ADVERTISEMENTS:
Similarities:
1. Pharyngeal apparatus.
2. Feeding and respiratory mechanism.
3. Origin and development of enterocoelic coelomic pouches.
Affinities # Branchiostoma with Urochordata:
ADVERTISEMENTS:
Similarities:
(Tadpole larva and branchiostoma):
1. Body shape and a fin-bearing tail.
2. Notochord extends nearly whole body length.
3. A hollow nerve cord above notochord.
4. Pharynx with endostyle, pharyngeal and epipharyngeal grooves.
5. Feeding and respiratory mechanism.
Affinities # Branchiostoma with Cyclostomata:
Similarities:
(Branchiostoma and ammocoete larva):
ADVERTISEMENTS:
1. Body elongated bearing a continuous dorsal.
2. Presence of an oral hood.
3. Velum guards the mouth.
4. Presence of endostyle.
Affinities # Adult Branchiostoma and Cyclostomata:
Similarities:
1. Presence of myotomes.
2. Presence of gill slits.
Dissimilarities:
1. Absence of vertebral column and paired sense organs in Branchiostoma.
Affinities # Branchiostoma with Vertebrata:
Similarities:
1. Presence of pharyngeal gill slits.
2. Presence of notochord.
3. Presence of dorsal, hollow, tubular nerve cord.
4. Presence of phosphocreatin and phosphoarginine, the sources of energy supply for muscular activity.
The old concept that Branchiostoma is a recent derivative of ascidians has been abandoned. Alternatively, Acrania may be a specialised and degenerate descendant of the Agnatha.
The evidence as to the real position of acrania is still insufficient, but investigations tend to bring them nearer to the crariiates. It is probable that the Branchiostoma and its allies, fish-like creatures existed in Silurian seas, and the vertebrates evolved from animals of that kind.