286
FINDLAY E. RUSSELL
are provided with small, pincer-like organs, the pedicellariae, which
are distributed over their entire body surfaces between the spines.
The pedicellariae are modified spines with flexible heads and serve
in keeping larvae and minute creatures, aa well as debris, from settling
over the body of the urchin or starfish where they might obstruct the
respiratory organs or the tube feet. Pedicellariae are most highly
developed and always present in the sea urchins, where four main
types may be found ; there is considerable variation in form within these
types.
The glandular, gemmiform or globiferous type pedicellaria serves
as a venom organ. In most echinoids the so-called “ h e a d ” of this
type is composed of three calcareous jaws or valves, each having 8
large basal portion with a convex outer surface and a concave inner
one, and each tapering into a delicate shaft curving sharply inward
and terminating in a fine, rounded, tooth-like fang. The tip of the shaft
is sometimes channelled, presenting the appearance of two lamellae
which merge to form the tooth-like fang. The jaws are usually invested
in a globose, fleshy and somewhat muscular sac which possesses a
single or double gland over each valve. The venom glands discharge
the toxin through ducts which open just proximal to the tooth-like
fang. In the asteroids the poison glands are contained in the concave
cavities of the valves and do not possess a muscular sheath. In Echinothuriidae is found a variant globiferous type pedicellaria, dactylous,
which reaches its highest development in Araeosoma, where each of the
four or five long jaws are topped by a disciform expansion.
According to Nichols (1962) the jaws of the pedicellaria open when
touched on the outside, and they may open still further if touched on
the sensory hairs inside the valves. If the appropriate chemical stimulus
is applied to the body surrounding the pedicellaria the jaws will close
and discharge the venom. If the stimulus is applied only to the sensory
processes inside the valves the poison will be ejected while the valves
remain open. Pedicellariae may become detached from the body of
the urchin and still continue to inject venom into a wound.
In some sea urchins poison glands may not only lie in the “ head ’’
but may encircle the stalk. The stalk, or stem, is attached to the
head either directly by muscles or by a flexible muscular neck of
varying lengths.
3. Holothuroidea
Some holothurians possess special defense organs, the Cuvierian
tubules, which arise from a common stem of the respiratory tree.
When these animals are irritated they emit these organs through the
FINDLAY E. RUSSELL
are provided with small, pincer-like organs, the pedicellariae, which
are distributed over their entire body surfaces between the spines.
The pedicellariae are modified spines with flexible heads and serve
in keeping larvae and minute creatures, aa well as debris, from settling
over the body of the urchin or starfish where they might obstruct the
respiratory organs or the tube feet. Pedicellariae are most highly
developed and always present in the sea urchins, where four main
types may be found ; there is considerable variation in form within these
types.
The glandular, gemmiform or globiferous type pedicellaria serves
as a venom organ. In most echinoids the so-called “ h e a d ” of this
type is composed of three calcareous jaws or valves, each having 8
large basal portion with a convex outer surface and a concave inner
one, and each tapering into a delicate shaft curving sharply inward
and terminating in a fine, rounded, tooth-like fang. The tip of the shaft
is sometimes channelled, presenting the appearance of two lamellae
which merge to form the tooth-like fang. The jaws are usually invested
in a globose, fleshy and somewhat muscular sac which possesses a
single or double gland over each valve. The venom glands discharge
the toxin through ducts which open just proximal to the tooth-like
fang. In the asteroids the poison glands are contained in the concave
cavities of the valves and do not possess a muscular sheath. In Echinothuriidae is found a variant globiferous type pedicellaria, dactylous,
which reaches its highest development in Araeosoma, where each of the
four or five long jaws are topped by a disciform expansion.
According to Nichols (1962) the jaws of the pedicellaria open when
touched on the outside, and they may open still further if touched on
the sensory hairs inside the valves. If the appropriate chemical stimulus
is applied to the body surrounding the pedicellaria the jaws will close
and discharge the venom. If the stimulus is applied only to the sensory
processes inside the valves the poison will be ejected while the valves
remain open. Pedicellariae may become detached from the body of
the urchin and still continue to inject venom into a wound.
In some sea urchins poison glands may not only lie in the “ head ’’
but may encircle the stalk. The stalk, or stem, is attached to the
head either directly by muscles or by a flexible muscular neck of
varying lengths.
3. Holothuroidea
Some holothurians possess special defense organs, the Cuvierian
tubules, which arise from a common stem of the respiratory tree.
When these animals are irritated they emit these organs through the