It exhibits very interesting case of sexual diamorphism with giant female and dwarf male without segmentation. Body divisible into anterior proboscis and posterior trunk.
Female is 5 cm long, ovoid in shape and green in colour due to the presence of a pigment called bonellein. The formation of the head is peculiar. The proboscis is very much elongated forming long spatulate in front of the mouth.
Proboscis is bifid measuring 60 to 90 cm in length and homologous to prostomium of annelids.
Grooved ventral surface of the proboscis is ciliated through which micro-organisms are collected as food. The mouth is found at the base of proboscis and the anus at opposite end.
Excretory organs are a pair of anal vesicles, one or two nephridia which act as gonoduct.
There is a closed blood vascular system.
Ovoid body is covered by minute papillae. The parapodia and the cephalic appendages are absent.
A pair of hook-like ventral setae are present near the anterior end.
Male is very small, tuberllarian like, ciliated and without proboscis. It has reproductive aperture, ejaculatory duct, sperm forming coelomic cells, seminal vesicle and intestine.
In early stages male enters the pharynx of female and lives permanently in nephridium after attaining sexual maturity.
Fertilization is internal.

IDENTIFICATION

It contains a bifid proboscis and all above features hence it is Bonellia. Traces of segmentation, coelomate, bilaterally, symmetrical.

Characterstics of Minor Phyla

On the basis of presence or absence of a true coelom minor phyla are divided into two groups.

(A) Minor Acoelomate Groups 1. Mesozoa, 2. Ctenophora, 3. Endoprocta, 4. Nemertinea (Rynchocoela), 5. Acanthocephala, 6. Nematomorpha, 7. Rotifera, 8. Gastrotricha, 9. Kinorhyncha, 10. Nematoda

(B) Minor Coelomate Groups 1. Ectoprocta, 2. Brachiopoda, 3. Phoronida, 4. Chaelognatha, 5. Priapulida, 6. Sipunculida, 7. Echiuroidea, 8. Brachiata. Some of the representatives of the above phyla are discussed below.