Body

Chaetognaths are bilaterally symmetrical organisms. Their body is divided into 3 segments: head, trunk, and postanal tail, which are divided by septa. Chaetognaths have an egg-shaped head and a mouth on their ventral side, and also possess grasping spines (4-14 per side, which may be serrated, Fig 1), which are located on the side of their head. These grasping spines, as well as their teeth (species either have one or two rows), are used to capture prey. When swimming, chaetognaths cover their head and grasping spines with a hood (Fig 2), as it is proposed that this gives them a more streamlined body shape, however this hood can be rapidly retracted prior to feeding (Ruppert et al. 2004).

Chaetognaths came in variety of sizes, for example:
Spadella boucheri is the smallest chaetognath. It has a maximum length of 1.3 mm.
Whilst one of the largest is Pseudosagitta gazellae. It has a maximum length of 105 mm.
(Thuesen 2009). 


Figure 1: 100x zoom of serrated grasping spines of a chaetognath


Figure 2: SEM of hood covering a chaetognaths head.