|Etymology:- From the Greek Priapos a ancient phallic god symbolising male generative power.|
Characteristics of Priapula:-
1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a true coelom.
4)Body possesses a through gut with an anus.
5)Body in two sections, a prosoma and a trunk.
6)Has a nervous system with an anterior nerve ring and a ganglionated nerve chord.
7)Has no circulatory but blood corpuscles exist in the coelomic fluid.
8)Reproduction sexual and gonochoristic.
9)Feed on other marine worms.
10)All live in marine environments.
The Priapulida, or Priapula are a small phylum (about 16 species known to scientists) of normally small worm like animals. They range in length from 0.5 mm for species of Tubuluchus to around 200 mm for species of Priapulus. They occur in most seas, both tropical and polar, at a variety of depths, from shallow coastal waters to as far down as 7,200 metres. The name Priapulida refers to the fact that the scientists who first named them thought they looked like a human penis, however they look more like some species of cacti and it would not be unreasonable to use the term Cactus Worms as their common name. All known species are marine and benthic (meaning they live in or on the sea floor).
The priapulid body consists of two parts; a prosoma, sometimes called an introvert, and a trunk. The prosoma can be drawn into the trunk which is why it is sometimes called the introvert. The prosoma has the mouth at its anterior end. The mouth has 5 spines, called scalids arranged around its opening. There are also rows of scalids running down the prosoma in most species.
The trunk also often has spines or scales and may have, as in the case of the genus Priapulus, caudal appendages (which look like a bundle of worms at the end of the animals body). These are suspected of having a respiratory function. The trunk bears 20 to 30 external annuli (rings) but there is no internal segmentation.
The body has a cuticle which contains chitin as well as other materials which lies outside of the epidermis. This cuticle is moulted as the animal grows. The body cavity of priapulids is a true coelom, or a pseudocoelom, depending on which authority you consult. Embryology is little studies in these inoffensive creatures, and it is a detailed study of this which will answer the question. Behind the mouth is a pharynx which leads to a short straight gut which ends in an anus at the posterior (tail) end of the animal's body.
There are no organs for gaseous exchange and no circulatory system, although in some species, such as Priapulus, the fluid which bathes the internal organs contains corpuscles which carry a respiratory pigment called Haemerythrin. Metabolic excretion is by means of protonephridia.
The sexes are separate and there is normally only one ovary or testis. Eggs and sperm are released into the sea and fertilisation occurs in the sea. The larva has a set of plates (made of cuticle) that surround it, it is possible that it lives in the mud like the adult, it is known to go through a number of moults before it takes on the adult form.
The nervous system consists of a circum-oral ring and a nerve cord running down the ventral side of the body. This cord has a number of ganglia along it from which nerves run out through the epidermis to the rest of the body.
The epidermis is supported by layers of circular and longitudinal muscles which lie within it. The coelom acts as a hydrostatic skeleton. However despite this adult Priapulids are not good at moving and have great difficulty burrowing back into the mud if removed from it. They move through the mud using the spines on the prosoma and the prosoma's ability to be withdrawn into the body.
Priapulids are all believed to be predators feeding on smaller worms they find in the mud, but some species such as those in the recently discovered order Seticoconaria live sedentary lives and have a crown of stiff tentacle-like spines, and may therefore feed on anything that falls into this crown.