Physical characteristics

The animals are bilaterally symmetric with a head, trunk, and tail. They are transparent: the internal organs can clearly be seen in living specimens and in specimens that have been well preserved in formalin. The body cavity is filled with fluid that is surrounded by muscles and tough membrane; a mul-tilayered epidermis covers the outer layer of the body. The head has a complex musculature that supports the grasping spines or hooks that are the most obvious and recognizable features of this animal. There also is a mouth, one or two rows of teeth, and eyes. The eyes have photoreceptive cells, and most species also have spots of pigment in their eyes.

The nervous system consists of six ganglia that are present in the head, a superficial cerebral ganglion, and two lateral ganglia at either side of the esophagus. The cerebral ganglion is connected to a ganglion present on the ventral side of the trunk; paired nerves extend from the ventral ganglion toward the tail. Very little is known about the functional roles of the different parts of the nervous system.

The animals detect the movement of prey with ciliary tufts present on the body. The body wall is folded in the neck region, which forms a hood that can be folded over the head to allow the animals to swim more smoothly. In some species there is a layer of loose tissue present in the collarette region of the neck. Chaetognatha have tail fins and one or two pairs of fins on the sides of the body; all fins are supported by fin rays.

The alimentary structure consists of the mouth, esophagus, gut (which can have paired diverticula), and anus. The anus is located just before the septum that separates the trunk and the tail. The ovaries are situated at the posterior end of the trunk in an opening present on both sides of the intestine, just before the septum. The tail contains the testes. Ripe sperm is stored in sperm packages within the seminal vesicles that project from both sides of the tail.

The warm-water species are generally smaller than the cold-water species, with adults between 0.12-5.91 in (3-150 mm) in size. The largest species, Sagitta gazellae, lives in Antarctic waters and reaches a size of 2.76 in (70 mm). The benthic species are the smallest arrow worms, with Spadella cephaloptera reaching maturity at 0.12 in (3 mm).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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